Starting Class (demo): Human Engineer
Starting Class (full release): Drell Adept. I annoyed more than a few people with the old reave sound, I'm sure. I moved over to the Justicar once she came out though. I just loved Reave!
Workhorse: It used to be the Justicar and Male Quarian Engineer. I'm pretty comfortable with just about any class at this point (I only hate the Talon Merc on 90% maps since the only aspect of him I find fun is laying traps), but the GI and vanilla Human Adept are my most trusted kits.
Favorite: N7 Paladin with a CSMG, cryo shield, and no snap freeze (for maximum tank). Far from optimal, but trying to make good use of that shield on platinum is just a fun challenge to me. Sucks that only the heavy melee reliably primes cryo explosions and that the armor de-buff is broke, but watching entire groups of cannibals freeze themselves by meleeing my shield just makes me laugh more than it should (freezing phantoms is a nice plus). Works completely fine on platinum, it just takes good judgement on positioning. I guess how quickly it punishes mistakes and the close range fighting with a shield reminds me a lot of dark souls, which I love.