Most Favorite to least favorite.
1. Zaeed Massani -- I seriously don't understand why so many people hate this guy. They say he has no heart and he's a self-obsessed ego-maniacal ******, but they probably have never used him outside his loyalty mission, where he does come off as such. Here's a clue as to why he was so bound and determined to kill Vido: He and Vido founded the Blue Suns, a protection agency based in the Terminus Systems. Vido started hiring Batarian terrorists, Zaeed didn't like this. They argued, Vido payed ZAEED'S OWN MEN VAST SUMS OF MONEY TO HOLD HIM DOWN WHILE VIDO SHOT HIM IN THE FACE THEN LEFT HIM TO DIE. AFTER WHICH HE WIPED ZAEED FROM THE HISTORY BOOKS AND HIRED THOUSANDS OF BLUE SUNS AND HAD A BATARIAN FILL ZAEED'S SHOES. This would make anyone ridiculously vengeful. (Personally, I kill Vido, if he got away he could just take another factory hostage and you'd be forced to make the same decision. Also, he's the leader of one of the worst gangs in the galaxy.)
Zaeed originally takes the mission for the pay, yes. But if you take him with you and listen to him, you'll find that (especially after Horizon) he starts to realize what's going on, and plans on stopping it with all his power. He can come off as heartless and rude/whiny at first, but if you listen to him, he's actually more like Paragade Shepard. In my opinion, even though Zaeed's DLC he's the most believable character. He has a great Backstory, Personality (Except for his mission, where his rage consumes him, not unlike Garrus.) And also, he's the most useful in combat, with his inferno grenade and his sniper rifle. If he doesn't come back for Mass 3 as a squadmate, or AT THE VERY LEAST you get to do an epic, badass mission with the guy, I will never purchase another Bioware game again (except for Mass 3 of course, can't stop in the middle of the trilogy.) Also, I couldn't believe it when I found out he can't lead either team, I can see the argument, But it's ridiculous! Even if he has NO ability to lead, he has ALL THAT EXPERIENCE! At least he knows what to do and how to survive, at the very least he could have lead the second time, they were holding position as a distraction squad. Being under heavy fire, long firefights and surviving crap like that is his specialty, they created the character, but Bioware never really thought it through.
2. Garrus Vakarian -- I do believe Joker said it best when he said 'Well it looks like Garrus managed to work the stick out of his ass, but now he's trying to beat people to death with it.' He's gone through the most dramatic personality change in the world. I like it. I don't like that his calibrations always keep him from talking to me though. Very useful in combat, sniper rifle and overload are great.
3. Mordin Solus -- Okay, what's not to love about an eccentric, fast-talking, energetic, half-crazy 'Scientist Salarian'? Seriously, he's one of the best characters in the game, no matter how many times I hear his jokes I always laugh, 'NEVER breach doctor-patient confidentiality!' One of my favorites, and if he doesn't come back for Mass 3 I will be sorely disappointed.
4. Tali Zora Vas Normandy -- This girl has gone from walking Quarian encyclopedia to cute and respectable and grown-up. She is a very lovable character and she's not bad in combat.
5. Grunt -- I like his sense of humor an his personality, I like Wrex better, but as far as replacements go Bioware did great. Grunt is great in combat, I love is random outbursts of 'I...AM...KROGAN!' and his status as team-tank.
6. Jack -- A lot of people don't like Jack's attitude and personality, but I do. Cerberus wronged her and used her for experiments, drugged her, tortured her, rewarded her for fighting. That would turn anybody into what she is at first. But as you dig deeper you find so much more, more reasons to why she is who she is, I wouldn't mind hearing the full story behind that vandalism charge tha Hanar got her with. She's my default LI, but I usually swap between her, Mirand and Tali, in that order. She looses points though, for combat inefficiency.
7. Jacob Taylor -- Jacob is kinda cool. I didn't like him at first, but over all of my playthroughs I've started to like him, enough to where I actually find him entertaining at some points. Points lost for treating femshep like a piece of meat and for only being good in combat for those who know how to use him.
8. Legion -- Legion is cool, definitly one of those characters that you kinda like at first, then he keeps growing on you. He's a Geth, he's an infiltrator (like me) he also has a combat drone. extra points awarded for an excellent robot dance.
9. Miranda Lawson -- Awesome appearances aside, Miranda isn't all that great, she's okay in combat and has the team stats boost but that's it. Jack is right to call her a Cerberus cheerleader. One of her best moments is when she betrays TIM for you. Cheerleading aside, she has an okay personality, if a bit serious and her romance plot is good.
10. Thane Krios -- I don't really see why Thane is such a big hit, he's pretty boring, kinda good in combat and his personality is practically nothing. Don't get me wrong, sometimes he's pretty cool and funny, but mostly he's boring.
11. Samara -- Oh, boy. So much wasted potential. Bioware created a character who's the most dangerous lawman (ha) the asari have to offer, chasing after the sex-vampire she gave birth to, she's one of tthe most powerful biotics in the galaxy... and she's monotoned, grammar perfect, and so boring it's like watching documentaries about corn processing or listening to a Buzz Killington story. Like I said, she's okay, but SO MUCH WASTED POTENTIAL! I was utterly disappointed with her character.
(I don't have Kasumi yet, but I'm assuming she'd be between Tali and Mordin.)