Garrus, Joker, The Volus on drugs that thought he was a god.

Garrus.  
 
More than any other character in the series, he's had serious character development and it was all thanks to Shepard, so it made me feel like those two would be essentially blood brothers. He was my main man throughout both games, never was even a decision he was always in my party. Normally I don't agree with what most people say about characters in video games, but Garrus is a really cool and intricate character. 

Garrus.  I had him in my squad as much as I could in the first and second 
Second would probably be Thane.   
Third Wrex 
I also really like Mordin, but I never used him.  It was great talking to him.  He died in the final mission though :(

Garrus all the way. If I had one lord and savior, it would be Reverend Garrulous the Third.

Mordin, followed by Garrus, followed by Kaiden

Legion, Thane, Mordin, Garrus and Tali ... those would be the Top 5

1. Garrus 
2. Wrex
3. Mordin
4. Thane
5. Liara

Garrus, Wrex and Tali -- although I enjoy Mordin's beautiful singing voice.

That's about as hard to answer the question "Who's your favorite best friend?".

Garrus

Conrad 4 Lyfe. 

Kaiden. Closely followed by Garrus. Closely followed by Mordin.

Tali & the Illusive Man

Garrus is my boy.

Garrus is the man, took him on pretty much every mission across both Mass Effect 1 and 2.

Thane Mordin Kaiden's unseen corpse

I blame my phone for that stupid formatting.

Haven't played ME1, and I'm say halfway through ME2, but Mordin so far.

Garrus was in Mass Effect and in 2 Legion I found him very interesting also he´s a R-O-B-O-T.

GARRUS! and Thane.

Mordin. By a pretty wide margin :D

Miranda for sheer comic value. By that, I mean that I laugh every time I take her on a mission and think to myself how painstakingly the developers worked at rendering her tits and ass. And you can't tell me they didn't develop a bunch of raunchy skins for her just for their own "personal use"...

Asari like Liara, Samara, and Benezia. (For more than two reasons) Get it?!!

Garrus is by far the most interesting character...and as a FemShep he is hilarious!

I've always found Dr. Chakwas to be one of the most interesting characters. Wouldn't mind to see her as a teammate in 3. 
In terms of team mates, Tali. She'll be the only character I'll have consistently on my team through all three games. Also, her accent's pretty cool. 

They each have their own merits, but I really enjoy the female Shepherd and Mordin. As for NPC's, I really enjoyed Aria T'Loak and I look forward to seeing more of her. From the book series, I really enjoyed Kahlee Sanders, though she is a bit bland. I felt that the toughness of Paul Grayson and the sweetness of Licelle (especially considering her parentage) was a good match, though it was cut too short too soon.

Legion > Mordin > Wrex > Garrus. That conversation with Legion's excuses (or lack thereof) when trying to explain why he's wearing Shepard's broken armor was AMAZING.

@Snail said:

Why did you not make a poll? Anyhow, Garrus.

There's too many characters to cover; he'd probably end up not satisfying everyone with a poll.

I really like all the characters, so I can't choose, but I want to give special acknowledgement to the previous Shadow Broker, because he is probably the most bad-ass (in a sense) out of all Mass Effect characters instantly outdoing the Krogan. I hope we can see more Yahg in the third ME.

Garrus > Miranda > Liara > Mordin

Garrus is my bro. He's that dude that deserves a bro-fist every day. And Miranda.. what can I say, I like my ice queens :P

How did no one say Aria? She's badass!
Also Garrus, Thane, Legion and Mordin.

Aria because she's voiced by Trinity and she's just a bad ass.

Liara, even though she was kind of more a damsel in distress in the first one, but redeemed herself in the second and third game since she became a bad ass and also came out of her shell more.

Grunt, do I need an explanation?

Wrex, do I REALLY need an explanation?

Garrus is the space bro that I wish I had.

I like Mordin in Mass Effect 2, but not so much in 3.

Literally everyone but Kaiden, Vega and the Reaper kid.

It may be a bit corny, but I liked the Shepard/Garrus best buddies relationship because it actually seemed genuine. Whereas on the far end of the spectrum you get that fratty douchebag James Vega bro'ing it up with you that comes across like it was conceived by a 17-year-old.

"Hey, we're just gonna have this, like, totally deep heart-to-heart man, while we have it out and punch one another, cos we're guys and that's what guys do, bro."

Seriously, who rubber stamped that character?

Garrus and Legion are the two best characters in the series.

Mordin, Jack, Liara

It's a pretty solid tie between Thane and Legion.

Garrus>Joker>Jacob>Mordin> and Kaiden because apparently I am insane.

Garrus is the teammate I spent the most time with, but that's more for his usefulness in combat than a genuine love for the character. I did like him, but not enough for him to be my favourite. Mordin is next in mind, but he felt like a Bioware attempt to recapture the magic of Minsc & Boo from Baldur's Gate.

I wish Urdnot Wrex played a larger role in Mass Effect 2 and 3, instead of the inferior Grunt and the non-Krogan James Vega, because then maybe Wrex would be my favourite. There weren't enough moments with him to dethrone my favourite...

Zaeed Messani! He felt like the most belivable and consistent character in Mass Effect, with my favourite voice acting in all of the Mass Effect games (RIP Robin Sachs). Zaeed has a great deal of personality and presence that I find incredibly well done. Very unfortunate that he was add-on DLC, which elminated any chance he had at a decent role or presence within Mass Effect 3.

James Vega is the worst. He's entirely insufferable. I never had the Renegade "shut up you idiot" interrupt I so desperately desired as Vega asked or suggested dumb stuff throughout Mass Effect 3. I believe that I read somewhere that he was made to appeal to players new to the Mass Effect series. It shows.

Just think, they skipped a Krogan squadmate for him - the Human Krogan. Bioware really had something against Wrex. (ME series spoilers regarding Wrex: First they kill him (canon), then they replace him with a clone, and then they replace him with an entirely non-Krogan character!) Argh, Bioware! Why?!

I really can't pick. Legion, Thane, Mordin, Grunt, Miranda.

Wrex, Miranda, Legion.

Mordin is my #1.

Mordin.

I know not a lot of people like him but I personally love Zaeed Massani. I do not know why maybe it is because his voice is awesome, his war stories,or that his is just a total space bad ass. Also my other favorites are Legion,Thane,Garrus,and Grunt. My least favorite is probably Kaidan that dude is just so boring.

Either Wrex, Tali, Garrus or Mordin.

I don't think I could choose just one.

Shepard and Garrus: The greatest bromance in gaming.

The fish.