All of these melee weapons have limited reach. I've found Energy Sword most fun to use and most effective. If you use melee while in Scout's Tactical Camouflage ability mode, do note you'll instantly drop the camo mode the moment you attack an enemy.
NOTES: This seems to briefly STUN an enemy on first hitting them.
NOTES: Disappointing in that it does NOT play a guitar riff upon a successful hit. :-(
https://youtu.be/SiRPANOYP08
NOTES: Awesome sword that's not AT ALL like a Star Wars light saber. ;-)
NOTES: Although Katana has the same Range stat as the other melee weapons I've found, it at least FEELS like it has a little longer range melee-attack. Perhaps that's just an optical illusion? :)
NOTES: Very satisfying "DOINK!" sound effect when hitting an enemy with Shovel. :)

NOTES: This distinctive-colored weapon does surprisingly high Damage over Time to enemies -- even in the Common variant -- and is very powerful, if you can keep alive long enough for the DoT to do its work. :)

CAUTION: At close range, enemies you have set on fire will set YOU on fire, so AVOID firing the Flare Pistol at enemies in close proximity to your agent.
NOTES: Per the name, this pistol has an always-on laser targeting beam to help you with aiming. Should be especially useful to controller-using players.
FIRING MODE: 3-round burst with each trigger-press
NOTES: As with any 3-round burst weapon, this will conserve precious ammo more than "continous automatic-fire" weapons. So it's a good choice if you find yourself starved for ballistic ammo.
NOTES: I'm disappointed this doesn't sound like Dirty Harry's .44 Magnum. Make it happen, devs! :)
NOTES: Despite the mundane name, this feels like a step up from the Detective's default pistol, at least.
NOTES: Cool concept/look, but these pistols don't sound like Spaghetti Western pistols, and they fire slowly at the same time, instead of a more interesting alternating fire. :)

NOTES: This double-barreled shotgun fires in a fairly narrow cone and is very effective. The 20% accuracy rating is actually pretty accurate in terms of other shotguns in the game.
(Stapler actually)
NOTES: This rapid-fire stapler actually uses shotgun slugs ammo, so I've placed it in this shotguns category. Effective, short-range weapon, but gobbles its slug ammo extremely quickly.
NOTES: Main advantage over other shotgun types is a faster firing rate.

NOTES: I got a "compare with what I have equipped" screenshot by mistake and will update this screenshot at some point.

FIRING MODE: 3-round bursts
NOTES: As with any 3-round burst weapon, it's useful for conserving ballistic ammo if you're having trouble finding that ammo type on a run.
FIRING MODE: Single-shot, extremely high-damage laser beam with each trigger press.
NOTES: Cautionary note -- Heavy Laser causes vehicles to EXPLODE with a single shot, so avoid firing this while standing next to a vehicle. :)

Submachine Guns in the game spray their ammo continuously while you hold the trigger button down. The various fire rate ratings will give you some guidance on which SMGs are more likely to exhaust your ammo sooner. It's usually best to fire these in short bursts, both for accuracy and to conserve ammo.
NOTES: Advanced usually means the bullets are "guided" to some extent, though I didn't use this SMG long enough to verify that.
NOTES: Bullpup SMG, along with Suppressed SMG, seem to have the highest accuracy ratings.
NOTES: Micro SMG's rapid firing rate makes it feel fun to use, but it chews through ammo quickly.
NOTES: Gangster SMG has a bit slower firing rate than Micro SMG, which makes it a better choice if you're running out of basic ammo repeatedly, a a bit better accuracy.
NOTES: In theory, the noise suppressor may draw less attention, though in practice, I find it difficult to tell if it's having any effects in that regard.

Suppressed SMG seems to have the highest fire rate and thus gobbles the ammo most like Pac-Man eating pellets. :-)

Ammo Type: Bullets
NOTES: This sports the most sluggish firing rate among the machine guns, which means it better conserves ammo even if you fire it continuously.
NOTES: Seems peculiar the bipod doesn't seem to make this any more accurate than the other machine guns.
Ammo Type: Electrical
NOTES: Requires a brief f "wind-up" time before firing, so make sure to allow for that.
NOTES: This appears to have the fastest firing rate among "conventional" machine guns, only trailing the firing rates of the Minigun and Gatling Blaster.
NOTES: The "Smart" means this machine gun's bullets are guided (!) and will veer towards your enemies to a certain degree. :) This will let you do some firing around corners.

NOTES: Don't get excited by the title -- this isn't MDK's hilarious "Tactical Nuke." This is just a more powerful than usual Fusion Beam weapon.
https://youtu.be/62AwE8bCKIk
FIRING MODE: Continuous, rapid fire while trigger is held.
NOTES: This rapid fire weapon devours plasma ammo like Pac-Man eats pellets. :-) Very powerful, though best used in short bursts.

NOTES: This launcher's guided missiles will veer towards the nearest enemies up to a point, but won't *wildly* go off course to keep tracking them.
NOTES: Great range and accuracy, but sluggish firing rate.
NOTES: This powerful weapon's incendiary grenades burst into lasting flames on the ground that cause Damge Over Time to enemies (and your agent) if they step into the flames.

As with the Proximity Mines launcher, it's useful for setting fire traps in narrow corridors, but you could also "spam" it in a smaller location and try to lure a boss into the flames.
NOTES: Extremely effective when you place the mines in narrow hallways and routes that enemies must traverse to reach you; however, if fast-moving enemies are approaching or chasing your agent, you likely won't have time to drop mines before enemies reach you.

For boss battles, other players have found it useful to "spam" as many mines in possible in a small location, and then just lure the boss into the pile of mines. :-)
NOTES: Rigged Circular Saw spits out circular saw blades that bounce off walls and environmental objects.
FIRING MODE: With each trigger press, fires a salvo of three guided missiles.

NOTES BEWARE of self-friendly-fire when firing this near walls or destructible vehicles. Although that goes for any missile launcher in this game, because this launcher fires a salvo of three missiles, friendly fire can be even more devastating to your agent.

These weapons direct a continuous, damaging beam for as long as you hold the trigger down, until its ammo type runs out.

Flamethrowers are true showstoppers but absolutely devour the matching ammo. Make sure you have a 2nd weapon using DIFFERENT ammo so you can switch over if your flamethrower runs out of ammo.
https://youtu.be/LmFdL1poHV0
NOTES: Note that while Atomic Flamethrower applies Damage Over Time to enemies, it does NOT leave any residual "atomic flames" on the ground, so don't worry about "walling yourself" in with flames on the ground. This doesn't work that way. :)