Tali, Samara, Garrus, Mordin, Legion then Liara post Shadow Broker. 
That isn't including the person who i love to shoot in the foot Conrad Verner.

Legion. Absolutely enthralled from the moment I met him.  
Joker, Garrus &Wrex all run in an equal close second (:

For the most part, the entire cast is great, but Liara is my favorite. The best character, however, is quite possibly Ms. Shepard herself. Also… mad props to Urdnot Wrex.

I was never a big krogan fan.  Neither Wrex nor Grunt appealed to me much.   
 
I spent most of my time with Tali, Liara (1 & 2) and Mordin.

Tali > Legion > Joker > Mordin > Kasumi > Morinth > Others

I'm a lady who fancies Garrus :)

 Tali

Tali > Garrus > Mordin

Garrus is just one chill badass.

A year later, and I still like Zaeed.

Wrex and Garrus. The Mass Effect 2 Garrus I should point out. But yeah, Wrex is effin' awesome.

I loved thane the first time I played, but Mordin is the best character.

KELLYGRRRRRRRRRRRRL!

On a serious note: 
 
Tali - I love the Quarians, I love Tali. Stuck with her for almost the whole game. She's just too cool. 
 
Shepard - As was said before. Just a really cool character both paragon & renegade. 
 
Jack - I really didn't like Jack. But that was a product of excellent character design & writing. I didn't like because I was against her ways and she made feel uneasy. But she also came through for me in the end.. 
 
Mordin - Probably one of the best video game characters in years. Very unique, very likeable. Stuck with this guy for practically the whole game, also. 
 
I could go on, Bioware writes such amazing characters.

Garrus, didn't like Mordin at all. 

Kasumi and Garrus.

Kelly.... 
 
Nah its really Mordin, Thane, Legion, and the Illusive man. 

 
Saren. 
 
YEEEEEEEEEEEEEEEAHHHHHHHHHHH 
 
Edit: Also Wrex, Grunt and Pressly.

Wrex, Garrus, Legion and Tali. Hated Jack and Jacob.

Tali is cute, Garrus is awesome and has interesting moral dilemmas (his history with Shepard can basically be his downfall or rise to greatness), Mordin is just SUCH A COMPLICATED CHARACTER (at first you find him awesome, then kind of funny and silly, but then it turns out he has some really cool backstory and a - repeating myself - moral dilemma behind him), Thane (the scene when you first meet him is wonderful in terms of visuals and style). Also: Wrex.

Miranda...obviously.

Wrex, Mordin, Anderson, Saren and Jack.
 
All great characters!

Tali.

Tali, that hips beat Miranda's genetically upgraded butt

Miranda, Mordin, Garrus, Joker... and I'm one of the few people that thinks Jacob seems like a cool guy

I'm gonna go with Sheppard.
 
Mordin and Tali would be my favourite non-Sheppard characters.

Garrus, Mordin, FUCKING COMMANDER SHEPARD
 
and Joker

Tali, Garrus, Mordin and what is probably a surprise element here, Subject Zero (it's definitely not my favorite character, but love it or hate it, I gotta admit it's a very well written one and that goes a long way).

Samera, wrex, and tali

Sheppard... I guess but not really...

Thane and Mordin are my fucking homeboys. 

Jack and Wrex

Garrus, Thane, or Miranda.

Liara, Mordin and Miranda.

Forgot Kasumi somehow. She's my favourite after Liara. 

Garrus and Wrex FTW!

" Garrus, Thane, or Miranda. "

Garrus, Miranda, Mordin

I like Garrus and Captain Anderson.

Got to love garrus! He's a damn hero. 
 
I never liked Tali. She got on my tits

my favorite non usable character is Joker, he's such a smart ass. other than that I like Grunt, Thane, Garrus, and my all time favorite so far is Kasumi. She is just very cool and I wish they would expand on her back story. Oh yeah and Shepard rocks as well but that's a given.
 
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Garrus for being a total pimp. Because Garrus is a fucking pimp yo

Zaeed.

Tali'Zorah nar Rayya for me.  
 
Ashley Williams was always an odd attraction to me, odd in the sense that she is a religious believer and as strong morals. I'm not religious whatsoever though I'd say I have some morals. Maybe that's why I liked the character so much. 

Garrus.  I thought it was great that he was back for the second game.

Tough choice. I can easily say that I like Mordin, Wrex, Thane, Garrus, Zaeed, Kasumi, Joker, and Kirrahe, but I'm not sure which is my favorite.

Favorite Male Character - Tied between Garrus and Thane.  i just can't decide. 
 
Favorite Female Character - Jack.  Damnit I want to know more about her!  What makes her the way she is?  Is there a softie under that badass exterior?

Legion > Mordin > Tali
 
Hearing "Shepard Commander" for the first time is priceless.

Mordin > all
 
I haven't played ME1 (although I watched a Renegade playthrough) but I am nearing the completion of ME2 and he's the only character I would say I love and wouldn't even bother playing ME3 if he wasn't in it.