The Defender looks out for the team. Armed with a Shotgun and a Gremlin, they provide support and occasionally devastating fire to any squad.
Healing is part of their skillset, with choices such as Saviour (heals are more powerful) vs Medical Protocol (you can heal at range) alongside more traditional choices (Revival Protocol vs Haywire Protocol)
HP : 1-0-1-0-1-0-1-0 (+4)
Aim : 3-3-3-2-2-1-1-1 (+16)
Hack : 45-5-5-5-5-5-5-5 (+80)

The Operator is a cool customer, operating at distance from the team, providing support with Sniper fire, or offensive Gremlin abilities. As with the Defender, the Operator uses both elements of his skillset to achieve his goals.
HP : 1-0-1-0-1-0-0-0 (+3)
Aim : 10-4-3-3-2-2-2-2 (+28)
Hack : 30-5-5-5-5-5-5-5 (+65)

The Titan is a defensive powerhouse, using heavy weapons & grenades to cause mayhem on the battlefield. With additional HP and lowered mobility though, they are not the fastest into fire. Many of their skills are focussed on surviving lots of damage, but they have a handful of skills concentrating on the heavier weapons.
(Complimentary Mods: War Suit Gives Bulwark. The Titan's defensive set makes him a great candidate for this ability)
HP : 2-1-1-1-1-1-1-1 (+9)
Aim : 3-2-2-2-2-1-1-1 (+14)
-2 Mobility, -5 Dodge at Squaddie

The Bomber does one thing very well. Blow stuff up. This class works well defensively with the ability to enhance Frost Bomb, Flashbang & Smoke Grenades, so it's not all about the explosions. With a limited supply of explosives, the Bomber is best on quick/short missions
as long drawn out firefights will leave the bomber with little to do.
(Complimentary Mods: Grenade Damage Falloff - tweaked. Explosives in XCOM2 are very powerful and GDF goes some way towards mitigating that however it's worth making the environmental damage, *or* the explosive damage a bit more powerful as GDF can completely nerf grenades if left untouched. Your mileage may vary.)
HP : 1-1-1-0-1-0-1-0 (+5)
Aim : 3-2-2-1-1-1-1 (+13)

The Scrapper is an offensive powerhouse with the ability to run to flanking positions and blow the hell out of the enemy. Rather than try and focus on gun or sword, the Scrapper can use both equally well. Woe betide anyone getting close to an angry Scrapper.
HP : 1-1-1-1-0-1-0-1 (+6)
Aim : 3-3-3-2-2-1-1-1 (+16)

The stalker is a stealthy assasin relaying on agility and misdirection to confound her foes. Striking from the shadows is what she does best, and her perks enhance not only that ability, but agility based skills too. Using a Rifle on the Stalker will actually make the class much more OP, so for a balanced experience, stick to SMGs.
(Complimentary Mod: Long War SMG pack)
HP : 0-0-1-0-1-0-1-0 (+3)
Aim : 3-3-3-2-2-1-1-1 (+16)
+2 Mobility, +5 Dodge at Squaddie

The Blaster is a ranged specialist with plenty of pistol and rifle perks to ensure a balnced use of both weapons. At higher tiers, the Blaster is an unparallelled killer on the battlefield, able to get off many shots in a turn.
HP : 1-0-1-0-1-0-1-0 (+4)
Aim : 5-4-3-2-2-2-2-2 (+22)

The Gunner is also a ranged specialist similar to the blaster, but more suited to the moniker of Debuff. With shredding skills and area suppression skills, as well as the ability to demolish the battlefield with lots of bullets, the Gunner reduces the enemy's ability to do damage.
HP : 1-0-1-0-1-0-1-0 (+4)
Aim : 3-3-3-3-2-2-2-2 (+20)