Spawns on all difficulties, on most stealthable missions, including Bank Heist, Jewellery Store and GO Bank

About Security Guards
The security guards are the run-of-the-mill rent-a-cops that you can find in any bank, mall or jewellery store. They have typically no armour and can be put down with ease.
These guys are more likely to cuff you or call for backup, rather than fight. But they will still fire off a few shots, ruining stealth if they do.

Variations
They typically wear grey uniform, with the Gensec logo, carrying a Chimano 88.
Though there are variants, such as:

• Grey uniform carrying a Chimano 88 or the compact 5 (only grey uniform with cap/hat uses compact 5)
  
• Red Gensec guards from Go Bank, who carry Chimano 88 or the Compact 5, and wear Red and black uniform
  
• Armoured Gensec guards that often patrol maps on higher difficulties. Carrying Compact-5 Sub or chimano 88
  
• FBI Guards from Firestarter, Day 2. who carry Chimano 88, and wear black suit and tie. Usually equipped with a flashlight on their chest.

How to deal with them
These guys typically don't cause you much trouble. Though on higher difficulties, they spawn in numbers, which may be more difficult to try and stealth.
Any gun will do, from the Gruber Kerz to the Thanatos. Headshots make short work of these guys. Though if you intend to stealth, i would advise either cuffing in an isolated area (such as on roofs or in alley ways) so that you still have body bags if necessary.

Need to know
Cuffing or killing these guys in stealth will trigger a pager response. You must answer this pager or you will fail stealth. Max number of pagers you can answer is 4. After that, the operator will not believe you, and the heist will go loud.

Spawns on all difficulties, on all heists except Shadow Raid

About Metropolitan Police
These are the first responders after a heist goes "loud". They typically show up in 2-3 squad cars, after 30 seconds of the alarm being triggered (never seen the police move so fast).
These guys don't require pagers, though they can break your stealth on certain heists (such as GO Bank, Mallcrashers or Jewellery Store), as they spawn during stealth as well.

Variations
The police are all regular "boys in blue". however there are some variations, such as:

• First response police, in blue uniform and carrying a Chimano 88
  
• First response police, in blue uniform and carrying a Bronco
  
• Armoured Police, carrying a Compact-5 Sub
  
• Armoured Police, carrying Reinfeld Shotgun

How to deal with them
These guys are probably the easiest to deal with, as they usually only spawn on the first assault wave.
Similar to the Security guards, they have very low health, and a few shots will take them down easily. Though on higher difficulties, they can spawn on later assault waves as "Bronco Cops" and they should be killed as soon as possible. As their Broncos pack a hard punch, that can leave you vulnerable.

Need to know
These guys are almost the same as Security Guards, except you do NOT have to answer pagers. So they pose less of a threat during stealth missions.
Bronco cops have much higher damage than regular cops, be sure to take them down first. As they pose the most risk

Spawns on all difficulties, on all heists except Shadow Raid

About SWAT
These guys are the 2nd line of defense and most commonly spawn on lower difficulties. On higher difficulties, they tend to disappear after a few waves and leave it up to the FBI.

Variations
There are only 2 variations of the SWAT:

• Blue SWAT carrying either Compact-5 or Reinfeld
  
• Armoured Yellow SWAT carrying Car-4

How to deal with them
These guys only spawn when in a loud game. You only have to worry about killing them.
They have middle range health, but weaker firepower than the FBI. which makes them easy to deal with. Especially if you use an LMG or similar high capacity assault rifle. (Car-4 or AMR-16)
They don't pose too much of a threat, unless they gang up on you. Or are accompanied by a Taser.

Need to know
On higher difficulties, they are often accompanied by a Tazer, or can team up into larger groups of SWAT, in order to swarm the player.
On higher difficulties, they deal the same damage as FBI, so take care of them first. Before moving onto the FBI

Spawns on all difficulties, on all heists except Shadow Raid

About FBI
The FBI act as the Hostage Rescue Units and most commonly at the end / between assaults. to protect and secure the rescue of any hostages you may have taken.
They have higher damage than the SWAT, but lower health, making them easy targets if you catch them before they get close.

Variations
There are quite a few variations of this unit, ranging through 3 unit "types"

Unit Type 1

• Blue jacket and cap, weilding Chimano 88
  
• They deal the same damage as SWAT, but have less health.
  
• These cops disappear during the early assaults

Unit Type 2

• Armoured vest, weilding a Compact-5 (See left)
  
• They deal much more damage than the first type
  
• Often seen with specials or in large numbers

Unit Type 3

• Grey shirt, weilding a Car-4
  
• Has less armour, but does MUCH more damage than any other FBI unit

How to deal with them
These guys are more heavily armoured, so headshots are advised. They also have a tendancy to group up and swarm the player, so it's advisable to have a gun with high fire rate, in order to take care of them down. In small enclosed areas, a shotgun may also be very useful to dispatch of the FBI quickly. I would recommend a secondary Locomotive 12G so that your primary isn't sacrificed.

Need to know
The FBI do more damage than SWAT, making them dangerous in groups.
This unit will untie / uncuff any hostages you may have, and rescue them from you.

Spawns rarely on lower difficulties, but can spawn on all difficulties, on all heists except Shadow Raid

About FBI Heavy Response
They are easily identifiable as the green heavy FBI, and are much stronger than the FBI rescue units.
These units set up such events as the ventilation gas on the bank heist.
They are standard units in Very Hard difficulty and above, so you will face many of them.
They don't have pre-defined "tactics" such as the other FBI units, but they are lethal up close, so don't give them a chance.

Variants
There are 2 very similar variants of the FBI heavy, both with the same health and damage margins:

• Green FBI with Reinfeld 880
  
• Green FBI with CAR-4

How to deal with them
Similar to the FBI 2nd variant, they are heavily armoured, but they do the same damage as the 3rd variant, which makes them a much higher risk. You must dispatch of these guys as quickly as possible. I would personally recommend modifying the Car-4 for assault. With high ammo capacity and high damage, you will make short work of this unit.

Need to know
These guys have the armour of FBI Variant 2, and the damage of FBI variant 3. Make these a priority over all other FBI
They do not attempt to rescue hostages as much as the other FBI units

Spawns rarely on lower difficulties, but can spawn on all difficulties, on all heists except Shadow Raid

About Special Ops Responder
These guys are considered to be the toughest "non-special" units, as they have a bullet resistant chestplate. The only things that can penetrate it's armour is a sniper rifle, such as the Rattlesnake or R93.
These guys like to push forward as quickly as possible, and can really pile on the pressure in numbers.
They are easily recognisable as their bulletproof "tan" armour shows them off from a distance

Variants
There are no variants of this unit, They all have tan coloured bulletproof chestplate, and carry a Car-4, for maximum assault.

How to deal with them
A gun that sprays the bullets around is a lot less useful in this situation. Unless they are high powered and can be set to single shot, for better accuracy (less recoil).
I would recommend guns such as the R93, so that body shots still score damage. It's always a good idea to have a Bronco or Locomotive seconday, as it will be invaluable in close range against these guys

Need to know
Their chest is bullet resistant, so aim for the head.
These guys push hard, and without mercy. Taking them down is top priority.

Spawns only on the DeathWish difficulty, on all heists except Shadow Raid

About Gensec Elite
These fine gentlemen are the best of the best. Gensec's Elite units. Only called in to clean up the biggest messes (that would be you).
They are heavily armed, with some weaponry even we don't have access to, and they do not take prisoners.

Variants
They all have the same urban camo armour, with only a few differences in their weaponry.
They can take one of 3 guns:

• Benelli M3 (real name)
  
• UMP 45 (real name)
  
• JP36

How to deal with them
These guys have the best firepower, and the best armour (not including the bulletproof tan). Your best chance for survival is to dig into a sheltered room, where you can return fire in safety, and not expose yourself too much to their onslaught. I would recommend a highly accurate gun, such as a Car-4 or R93 to deal with these guys.

Need to know
They have guns we don't even have access to, so you know they are badass. Use extreme caution when confronting these guys.
High range weapons such as sniper rifles are ideal.

Spawns on all difficulties, only on the Shadow Raid heist

About Murkywater
Making a return from PAYDAY: The Heist, these guys only got more badass. They are a heavily armoured Private Army. They can take a beating, withstanding as much damage as the FBI Heavy responder. These guys are similar to the security guards. As you have to answer a pager if you were to take down one of them.
Only appearing in a stealth based heist, it makes them easily killable by a single strike, whilst you remain undetected (just like the Bulldozer in the Train heist).

Variants
There are no variants in the Murky's appearance, though their weaponry varies:

• UMP 45 (real name)
  
• Car-4 (After the Big Bank update, this gun is no longer user)
  
• Custom MurkyWater Eagle Heavy Rifle

How to deal with them
Despite the high level of armour and damage, these guys can usually be easily killed whilst undetected, as it only takes a single shot to put them down. However, if you are detected, it can take quite a bit to kill them. So i recommend a strong assault rifle, that is still suitable for stealth. (such as the ever popular, Car-4).

Need to know
The Murky's are easy to deal with when undetected. So stay low, stay dark, avoid guards and gunfire.
You can treat these guys like glorified security guards. With a lot better weaponry.

Spawns on all difficulties, mostly on Hectors (drug related jobs). Also on Big Oil Day 1

About Gangsters
You're on the wrong side of the tracks when working with these guys. Though they are relatively easy to kill, they have very strong weapons that can take you down in a few short seconds. So if you have to fight them, put them down fast!

Variants
There are 4 variants of of the Gangsters. Though they are very similar. There are some key differences:

• OVERKILL MC:They are a biker gang found on Big Oil day 1, notoriously suing a Swedish game development company for stealing their name. These guys only pack Chimano 88, and are of little threat.
  
• The Cobras: This is a street gang, dressed in red. Most commonly found on Rats day 2, during a meth trade, but infrequently found on Big Oil day 1, trading with OVERKILL MC. The Cobras all carry Mark 10 Subs
  
• The Mendozas: These guys dress in casual wear, and are the rivals of Hector. They appear on day 1 and day 3 of Rats, as well as day 1 of Firestarter. They also carry Mark 10 Subs
  
• Russian Mob: They are Rivals of Vlad, and dressed in suits on the Nightlclub Heist. They carry Mark 10 Subs.

How to deal with them
These guys are by nature, very easy to kill as they have no armour at all. Though they have a high damage output. So it's always good to kill them first, in any way you see fit.
When stealthing, you can covertly kill them in one punch / shot.

Need to know
These guys are comparable to security guards, though you do not have to answer pagers once you have dealt with them. So have at them.

Spawns on all difficulties, on maps with high vantage points, such as Bank Heist

About Snipers
These guys are lethal. They can decimate a players armour in a single shot, making it very dangerous to venture outside without checking the area first. Fortunately, these guys have rather poor armour. So it's easy to take them down with an accurate weapon, such as.. you guessed it. a Car-4, or sniper rifle.

Variants
There are 2 key variants in the Snipers, the SWAT and FBI snipers

• The SWAT is dressed in blue
  
• The FBI are dressed in green

How to deal with them
Due to the Sniper's poor armour, it is very easy to kill them with even the weakest weapons. You just need to ensure you have an accurate gun in order to take them down, before they take you down. They tend to miss the first shot (if you're moving), which will give you time to find and kill him, before his second shot.

Need to know
They tend to miss their first shot. Use this to your advantage
They always spawn in predesignated locations on the map. It pays to learn the locations and plan accordingly.

Spawns on Hard Difficulty and above, on all heists except Shadow Raid

About Shield
These guys are probably the easiest special to overcome. Though they usually spawn with protection, in order to get close enough to pose a serious issue, if you let them.

Variants
Much like the sniper, both the SWAT and the FBI dispatch the Shield unit. With a few differences.

• The SWAT Shield is dressed in blue, with a black shield. They carry Chimano 88.
  
• The FBI Shield is dressed in green, with a grey, lighter shield than the SWAT. They carry the CMP Sub.

How to deal with them
The shield is probably the easiest special to deal with. They have reletively weak weapons, and pose little threat offensively. The easiest way to deal with a shield is to use a sniper rifle's penetrative power to shoot through the shield. damaging the unit behind. Or to attempt to kite behind the shield, and shoot him from behind. Any gun will be suitable for this situation.
Another method is to hurl a grenade behind, which causes him to spin around, exposing his back when stunned. However, this is often frowned upon, since it may kill civilians, break boarded up windows etc.

Need to know
Killing a shield can often be benefitial, as the shield often stands. protecting you from oncoming fire.

Spawns on all difficulties, on all heists except Shadow Raid

About Taser
The taser is the troll of the police force. This unit will force you to put all your ammo into the sky, whilst making you do the Harlem Shake. You must kill him immediately.

Variants
There are no Taser variants, this guy doesn't need them. He's pure, unadulterated awesome.

How to deal with them
Despite how epic these guys are. They have relatively strong body armour, so it's advisable to aim for the head whenever possible. Against the tasers, it's always helpful to have a single fire gun such a shotgun or a sniper. So that your ammo isn't deposited into the floor too quickly.
Often the Tasers will spawn and assault in pairs. Which makes it very risky for you to take them on solo. I would advise having a high powered sniper Like R93 or Thanatos. If you are able to kill them from a distance, it will make it much less likely for them to be of risk.

Need to know
They often spawn in pairs, caution is advised.
Tasers are awesome.

Spawns on Hard Difficulty and up, on all heists except Shadow Raid

About Cloaker
After much anticipation. The return of the cloaker (you asked for it folks). This guy is widely known throughout the community has being the biggest mistake they ever asked for.
This guy has the ability to down you with the flick of their pinky finger (or a flying kick).

Variants
Much like the Taser, the cloaker is too badass to have any other variants. Enjoy.

How to deal with him
You will hear the Cloaker before you ever see him, if you have a surround sound set up. You have the advantage. Though this guy is a mastermind when it comes to hiding (most of the time).
The key here is to NOT run away. This almost never works. He is faster than you, and doesn't run out of stamina. The only way to successfully handle these guys is to destroy them with a high powered assault rifle.
They attack too quickly for shotguns or Broncos to be effective. So i advise a sniper, or your trusty Car-4 to mess him up.

Need to know
You will hear him before you see him. Listen for the ringing / buzzing noise. You will know where he is.
Running away is useless. don't do it
Aim for the head. The critical damage he suffers from that is greater than usual.

Spawns on Hard Difficulty and above, on all heists except Shadow Raid. However. A Bulldozer is able to spawn on Rats Heist, on any difficulty. Jumping from the SWAT Helicopter

About Bulldozer
This is it. If you've ever wondered "what's that thud from around the corner?" This is him. The Bulldozer.
This guy is slow, but if you don't watch your back, even the weakest Bulldozer can cause some serious problems for the whole team.

Variants
There were several variants to the Bulldozer, including a seasonal exclusive

• The Green Bulldozer, he is the weakest of them all. Though he is still formidable and very dangerous up close. He wields a Reinfeld 880 Shotgun
  
• The Black Bulldozer (SaigaDozer), he is the middle range Bulldozer, who is in every way improved from the previous. He has better armour, and improved weaponry. He wields the IZHMA Shotgun with a drum mag (not available to us)
  
• The Elite Bulldozer (SkullDozer), this guy is the real deal. You wanted a challenge, you got one. He wears urban camo armour, similar to the Gensec Elite, and wields a KSP Light Machine Gun
  
• The Headless Bulldozer, he is a Halloween Exclusive Bulldozer from the Safehouse Nightmare. He is visibly similar to the SaigaDozer, however he has no head and a sword on his back. The lack of a head makes him harder to kill, as he has no weak spot. He wields the same IZHMA shotgun as the SaigaDozer

How to deal with them
Without a doubt, the easiest way to take down a Bulldozer is to use the Thanatos .50 Sniper Rifle.
This sniper has the potential to take down the GreenDozer in 1 shot, the SaigaDozer in 2 shots, and the SkullDozer in 3 shots. Assuming you hit them in the head each time.
If you are unable to snipe the Dozer from a far, your next best option is to get in close with an assault rifle. The assault rifle has to be high damage and high fire rate. As it's only a matter of time until the Dozer kills you, if you're too close.

Need to know
The Headless Dozer was the introduction of the SaigaDozer
Do NOT use a shotgun on a Dozer, it won't end well
Using a grenade or ECM feedback is an excellent way to stun a BullDozer, and make them an easy target for a few seconds. Syncronising greandes can make even the SkullDozer easy work
Cheney? good luck buddy.

Credit to the PAYDAY wiki for the photos used in this guide. As well as some of the information.
Thank you to Sandman who has provided more information
Thank you to Cloodden who has corrected some spelling and information

Thank you to the Car-4, you're awesome!