Tommy Vercetti. I thought he was easily the funniest and most badass of the GTA protagonists. Plus, being voiced by Ray Liotta was never going to be a bad thing. On top of that, I just found him to be the most likeable of all the GTA protagonists we've been given.
In second place would probably be Johnny Klebitz but for different reasons. I thought the biker scene made a brilliant addition to GTA and it was awesome to actually get to play as one of them. Looking back I think the only other time you got to interact with a proper MC was back in Vice City when you had to do some missions for Big Mitch Baker and The Angels.