Probably... Garrus Vakarian, Tali'Zorah Vas Normandy, Anders and Aveline.
... There's probably more. Like Samara and Merill, and Zevran.
Garrus - Because he's just a very enjoyable character. He's a batman-style badass, but he's not totally infallible. There's a lot of drama surrounding him too which is really fun to plunge into and either let him indulge in his psychotic revenge or take the longer route of trying to lay his demons to rest. Plus, as a gay gamer, Garrus is intensely attractive despite being a weird alien bird thing. Must be the voice. Or badassery?
Tali - Probably because it's easy for me to relate to the "coming-of-age" story character, but Tali first grabbed my interest as nothing more than a conduit through which I could learn about one of the most interesting and scarcely represented species in Mass Effect. But in the second game she started to come into her own and she was no longer just the fellow introvert who I was attracted towards, but she had a really interesting story of a neglecting father and the politics and social dynamics of her people.
Anders - What can I say, I just love Anders. He's a tragic character, chasing after peace and understanding, all the while being lead back to violence by either his own extremist ways or the demon that resides within him. As the game progresses, of course, we learn that there are two demons within Anders that he's been fighting all along. Along the way he's a charismatic healer always making humorous observations - The Allistair of Dragon Age II.
Aveline - I doubt many others will pick her, but I loved Aveline. She was always the voice of reason. Yes, she was the captain of the guard, the representative of lawfulness and loyalty, but still she ran with my Hawke, a rogue, and two apostates, Merill and Anders. Instead of constantly making offensive comments like Fenris, she saw her comrades good intentions and abided by them. Not that she was spineless, either. At times she'd put her foot down and I understood her place. Getting Aveline her promotion and helping her with her romance was incredibly fun and I truly felt like I had built a friendship by the end of it.
