Wheatley, I really liked him. He was cynical, subjective, funny, timid, stupid, lighthearted, and pure awesome.
If I could pick 2, Chell'd come 2nd.
For a character who doesn't talk, she's got a lot of backstory.
We learn that she was one of the smartest people at her school.
We learn that her parents abondoned her.
We learn that she was taken to the Apaerture Science "Bring your daughter to Work day" by her
foster mother.
And much much more stuff I haven't mentioned 'cause I don't want to spoil it.
She's backed up by a shit-ton of personality, which makes her impossible not to like.