As many of my previous answers have eluded, I like to take my roleplaying quite seriously, and so it’d have to be someone serious. I’d love to say that it would be someone fun or something ridiculous, like Tasslehoff Burrfoot or the great HYPNO TOAD….-
But, in reality it’d be someone like Raistlin Majere from Dragonlance. I think Raistlin would enjoy the challenge of a good roleplaying game, I think he’d take it seriously and he wouldn’t accept shenanigans from other players. I also get the overall feeling that he’d probably make quite a good Keeper or Storyteller. The only downside is that he might end up working against the party but I suspect that he’d have a very detailed and interesting story behind why that was the case.
If it wasn’t Raistlin, an established fictional character, it’s be my own NPC, Trebor Minntt. It’s easy to explain why, he’s basically my avatar in Planescape and so I think we’d probably play well together but that seems like the very easy answer.