I can't pick just one.
Kaidan, I liked everything about him. I liked his personality, his kindness and yes, the romance. In ME3 I loved hearing him ask Shepard to wake him up next time.
Garrus was my best friend in ME2. We blew up things together and the one time I did his romance it was sweet.
Jack was my surprise character. I did not like her the first couple of times I played. But then she died saving Miranda and then I was nicer to her, even supported her over Miranda a few times. Then one of my few male Shepards romanced her, was kind to her and I really REALLY enjoyed seeing her developed into someone I actually liked.
Mordin became one of my favorites when he sang.
I actually liked all of them. But these 4 were my favorite out of everyone.
In ME3, I enjoyed talking to Steve and Sam, sort of wish I had met them sooner.