That question is [i][b]so[/b][/i] difficult to answer. Because my favorite characters all share the same trait, that so few partners of mine seem willing to nail. But the RP's I did with this person all had multiple characters in them. (Making choosing [i][b]one[/b][/i] a near impossible task.) But the characters themselves were all strengthened in the same way [i]most[/i] fictional characters are.

Being a [b][u]proactive[/u][/b] protagonist, that knows how to deal with the problems they face. (I play the villain a lot. Because I enjoy killing/defeating my darlings more than most do. [i]As more than one of my group RP's have ended, because a fight had fairly hurt someone else's character.[/i] But not one of those quitter's truly had no options left. They simply lacked the creativity or drive to do better.) 

And it feels strange that so many characters in roleplaying are almost always strictly reactionary characters. So worried or reluctant to handle curveballs that I throw. That it's a rare and glorious moment when someone gives [b][i]me[/i][/b] a challenge. But I love that type of creative problem-solving that a good D&D campaign thrives with. And even more so in a 1x1 setting, where two people are perfectly playing off each others characters.

So without answering with an exact character/name. [s]Maybe that's cheating.[/s] The [b]characters[/b] that I felt she played so well. Were enjoyable in part, because each one [b][i]felt[/i][/b] like a real character. But I suppose it's extra satisfying, when you get to watch a particular partner's character kill your "evil bastard" character, and does it in a clever way as well. 

So for my most recent example. I suppose it's the character that let me see my darling die. :P
[hr]
Now on the opposite side of this question. 

Can you think of someone else's roleplaying character that was memorable [b][i]for all the wrong reasons?[/i][/b] The type that's hilarious in hindsight. (Surely someone has a good story that they want to tell.)