I've played GTAV, and first i was sure Franklin will be my favourite, but after some time, i realised that Michael is the best, anyway, if i will ever make a game, i'm sure i wont be able to make such a good one!
Trevor's last line makes me LOL every time I watch his trailer, as for Micheal he seems pretty awesome but on the screenshots he looks a lot older than on Trailers. And if I my sources are correct then Franklin's Last name is Johnson, that should give you guys something to think about :D
Lets see: Michael is rich and owns everything; Franklin is a gangsta and as Florin said, he's kinda like CJ; and Trevor is THA BOSS. So, my favorite would be .... uhm .... Franklin =D