I think that Michael is my favourite and Trevor is a close second. You can see that Michael treats franklin like his son because in one mission Michael pushes franklin on the train first (then jumps on himself along with Trevor). His family aren't very nice with him really but it gets better ish in the end. The conversations between Trevor and Michael are just awesome and hilarious. I do feel sorry for Michael because he cares about his family and wants to keep them safe, however he nearly had Trevor killed and made Trevor think he was dead (kinda bad but you can see he was trying to protect his family but he shouldn't have tried to get Trevor killed).
Trevor is an insane kinda guy, he can be hilarious and kinda psychotic but you can kind of understand why he's like that (can't really explain it but I read something about his dad being cruel to him as a child?) he is quite loyal to Michael as he got a tattoo on his left arm saying 'rip Michael'.
Franklin is the one I don't really get, I wish I liked him as much as the others, I think he's in it for the cash and a better life (fair play to him). He doesn't have as much of a background story like Trevor and Michael. Maybe if franklin was part of the trio from the start of the story instead of brad (or included with brad) it might change my opinion or if he had more of a background story. I do like how franklin holds the trio together near the end and I also like chop. If Trevor or Michael had chop I would probably not want to play franklin at all and feel forced to play him.
If any information is incorrect I do apologise