Garrus and Wrex, hands down. I can't decide between them, but I love them both.
You know what's best about them though? They're so freekin' alien. I love it. I mean, when it comes to choosing your squad in Mass Effect there are 2 humans, a bio-hazard suit that may as well be human, and a human with blue skin and in-built corn rows. The only true aliens you can pick are Garrus and Wrex, and they're both badass. The way Wrex says "...Shepard" after you finish speaking to him always makes me smile.
Of course, when it came to the firefights, they were pretty useless. Which is why I also vote for my entire stash of ammo in Stranger's Wrath.