I'm surprised so many people like Jack, I for one can't really stand her. Ashley, Kaidan, Samara, Mordin, and Jacob are eh. Not a big krogan fan.
Liara is my love interest from ME1 so she's up there. Miranda's a decent character.
Never got the DLC, so I'd say very close between Thane, Garrus, and Legion.
On a character level nothing beats Garrus. He has an arc no other party member has (partly in due to being in both games) and he evolves from the first to the second. Events change him into a character he wasn't when you first met him. And the dynamic between him and Shepard is unprecedented in comparison to how other people are with Shepard. He just feels a lot more fleshed out than the other ones and he doesn't whine about stuff, which helps.
Liara became (by accident actually) my love interest in ME and that actually turned a lot more interesting with how the second game unfolded (especially with the Shadow Broker dlc), so she is also quite a personal favorite of mine.
ME2 have a lot of really cool characters that are really fleshed out in their own right but while they are so detailed, they don't have the history that those from ME have, or at least those that I enjoy.
Honorable mentions; Legion, Mordin, Grunt & Wrex.
Liked; Thane, Kasumi, Samara.
DIsliked; Ashley, Kaidan, Tali, Jacob & Miranda.
If we're talking Antagonists/Allies I really think Saren is at the top of the list with Sovereign and the Illusive man close behind.
I love the very fact that BioWare's games have large casts, even if some are a bit cliched.
I feel that Mordin is the most well-developed and portrayed character, that appeals not only to me but to a great deal of other people as well.
Personally, I like most of the ME characters. The only ones I disliked were Jack (trying way too hard to be edgy there, BioWare) and Jacob, but that's only on an emotional level. They are all fleshed out and played well.
Tali is awesome. Garrus is just cool.
And in ME 2, Miranda was pretty much the perfect squad mate. Useful in combat, had connections, ready to hook up when necessary.
Oh, and Mordin. Wish he survived my mission : (
Oh, and Thane. Damn, so hard to choose, lol.
Wrex, Legion, Mordin, and Garrus. Garrus was probably the party member I used the most. He was also one of the four dudes that died on the suicide mission. That was a giant "NOOOOOO!!!" moment and, funnily enough, made the ending better to me.
Wrex is the fuckin' shit. I didn't even play ME 1 and he's still my favorite character. He better have a bigger role in number 3.
Other than that Legion, Wrex, Mordin and Liara. Hackett comes on a corner for having a sick voice.
Legion and Wrex are just badasses.
Mordin is sort of witty and have a story thats interesting to me.
Liara saw some character evolving between ME1 and LotSB, yet stayed in her intellectual persona.