I tend to cycle between Infiltrator (Geth, Turian, and Alliance A.I, sometimes salarian and base character), Soldier (Turian, N7 and Vorcha) and Engineer (Geth, base character, salarian).
Infiltrators have diversified incredibly (and possibly the most out of all the classes) in Mass Effect 3. It used to be a case of the one-trick pony (admittedly one of the most fun and useful one trick ponies in the game
), where all there was was Tactical Cloak and a powerful sniper rifle. Previously you could try and go the caster route (fire one power before breaking cloak) but generally with only Cryo Blast or Energy Drain available your versatility was limited to 2 health types. With the release of the Quarian, Drell, Turian and Alliance A.I, Infiltrators have diversified into not only more caster types, but also diversified past the obvious sniper rifle specialisation (assault rifle infiltrators with the turian, shotgun infiltrators with the Alliance A.I.)
Infiltrators are the specialists in putting down 3-4 figure damage to single targets. How quickly you can do that as a player determines how good the Infiltrator is.
Not a lot to say about the Soldier. Good, all-purpose damage dealer with a weapons-centric (mostly assault rifle) focus with some very good casters in the mix and probably the best suppressive ability out of the classes. You'll see me swear by the Proximity Mine (Turian Soldier) as one of the best skills in the game period, both as a support skill (20% damage bonus to all damage recieved) and as a heavy damage dealer (1-1.5 meter radius, 800-ish? I think? damage). N7 Destroyer is incredibly solid with a good machine-gun type of assault rifle, as there are ammunition capacity, accuracy and damage bonuses all-round as well as an automatic missile launcher or multiple grenade launcher. It's fun all-round in Devestator Mode when you will never be staggered by any heavy attack by an enemy, and it's... empowering (to put it lightly) to slowly close in on a Brute even as it charges at you
Vorcha Flamer OP nerf pls
Engineer is the quintessential caster support, once able to hold their own as a damage dealer but now pretty much outclassed by other new class characters that are much better at the job. Offensively there's a heavy focus on stumble, stagger and stun animations, as well as set-ups for those incredibly important combination detonations - or as I like to call it, a song of Ice and Fire.
With the baser classes, Combat Drone is a guy you'll become pretty attached to, putting out decent damage at all ranges and helping you score those tech combos when all by your lonesome. There are some great grenadiers like the Quarian Male and Turian, and I've seen N7 Demolisher Widow builds, so with the unrestricted weapons choices for Mass Effect 3 these things aren't set in stone. You could very much have an Engineer putting out enough damage to go toe-to-toe with an Infiltrator, if you play your cards right.
The only Sentinel I bother with is Vorcha, because he's a great (and oddly unique) HP tank, but more towards active regenerative ability (which unlike Tech Armor doesn't mess around with cooldowns as much, a Sentinel's great weakness). Also, Flamer is a top, top, top tier tech ability that not only does a huge amount of instant damage but persistent damage over time that stacks. And also paired off with another flamer can pop off a flame explosion virtually every second
Biotics aren't my thing; That's Lex's speciality, really.