1.
Conrad Verner HK-47 - Come
on, who doesn't? It's almost tempting to leave him out for the sake of uniqueness, but that'd be disingenuous since he's an awesome character, period, and as the Elcor have proven, funny verbal communication automatically makes the dialogue twice as interesting (especially if four out of five lines are asking for permission to kill people). I fell out of my chair laughing when I first used him as a translator. You know the part. After that, I was smitten.
2.
Jolee Bindo - I'll admit, I didn't care for this guy at all when I met him. His hippie-ness and condescending attitude led me to believe he was just going to be a straightforward Jedi geezer... but after his story about the Force Prodigy and the starship drive had me in stitches, Bastilla spent the rest of the game on the Ebon Hawk.
3.
Alistair - Sure, he had a few Onassi-like tendencies ("I DON'T TRUST YOU AND I DON'T WANT TO TALK ABOUT IT"), but he was an absolute breath of fresh air in Dragon Age. Everything up until that point had been so grim and super-serious that watching him antagonize a mage was probably the first time I'd seen genuine character on display. I've always had a thing for smartass-with-a-heart-of-gold characters, and his often-horrified exchanges with Morrigan were simply priceless. It was always nice to have him around to lampshade every situation that he wasn't personally invested in.
4.
Tali'Zorah vas Normandy - Dem hips. Dat pseudo-Serbian accent. Dat fumbling, awkward vulnerability. Yeah, everything about her is made to appeal to my latent patriarchal tendencies, and I hate to admit that it's embarrassingly effective, but she manages to be a strong, interesting character nonetheless.
Also: hips.
5.
Legion - I don't know if my two favorite Mass Effect characters are a quarian and a geth by coincidence, or because I've always found the Quarian-Geth war to be much more philosophically interesting than the "universe ending cuz badguys" main plot, but either way, one thing's for sure: Legion was amusement incarnate. I could analyze the strengths of his character, but it'd just be easier to say he was thoroughly unique and very, very well-executed... even if it would've been nice to pick him up earlier in the game. I mean, after all that buildup and mystery surrounding the Geth, having a walking Geth encyclopedia show up tends to be distracting in the best way possible.