Hmm, tough choice. They all have their ups and downs, pretty much.
Probable favorite: Legion - Both the whole mysterious Geth race - you know, the part of them that aren't trying to kill you - and a rare individual among Geth who has a story of its own. It wins... though you could say it cheated having all that going for it.
Probably second favorite: Mordin - His quirky nature makes him really likable. If Legion didn't cheat, he'd be on the top of my list.
Probably least favorite: Samara - I sexed her but, you know, what's there to like about this character? "Hi, I got perfect genes. I smart enough that I headed the project that brought you back to life. Oh, and I'm also a complete ice queen and murder people I don't like without blinking an eye."
Maybe if my other female choices weren't the loose intern, the tin can, the alien paladin, and the tattooed train wreck, I'd have got my Paramour achievement somewhere else. Of them, I think Tali's the most likable (read: sane) of that list, but I'm still not sticking my virtual wang in that. Screw Bioware's writers for trying to spin, "Oh, but she's a nice girl, you shouldn't hold that against her" into the story. It's whacked enough to get it on with aliens, nevermind those who have an known immune problems. Maybe in the next game we can hit on Krogans with AIDS.
Probably second least favorite: Jack - "Rar! I'm a bad girl with a bad history but I'm a super-duper biotic!" I sympathize, lady, but I've got a job to do. Should have left her in the box I found her.
Disqualified: Zaeed. Nice voice work, but otherwise he's a downloadable character who feels like a downloadable character. He doesn't have proper dialogue and I could easily have got through the entire game without him. Gee, thanks for the merc, Illusive Man, but you shouldn't have. Plus, it felt like I was getting dragged by the nose through his loyalty mission.