Michael, Trevor, Franklin.
I like them all to be honest but Michaels story grabbed me the most. Trevor was crazy in a way that sometimes I could get behind and sometimes actually scared me a little. Franklin, while fine, was just a bit whiny tbh.
I like them all equally. They are all three pretty great. But I went with Trevor, Franklin, Michael. Trevor first because he makes me laugh every time I play as him. Franklin next because he has some really cool stuff and some funny stuff too (plus Lamar with either F or T is some of the best stuff in the game). Micheal last because of all the lame family crap.
Micheal, Trevor, Franklin.
I really enjoyed all the Family drama with Micheal's family and Jimmy is pretty hilarious. Trevor is just completely insane and I love it. Franklin is a good character but I didn't get into him as much as the other two story wise. I guess I'm just not a big fan of the gang stuff.
Michael, Trevor, Franklin
I really love the story with Michael and his family. It also had a perfect conclusion. Trevor on the other hand was a psycho and totally fucked up. He was a very interesting character because you never knew what he would do next. Frankling was your normal GTA character he was totally fine but he did not stand out as much as the other two.
Michael's probably my favorite. Franklin and Trevor go back and forth. Trevor's funny and his unhinged behavior is a breath of fresh air when faced with smug assholes like Haines and Westin, but he can be a dick sometimes. Franklin just seems inconsequential to the story once the Trevor/Michael drama gets introduced. Basically, Franklin is cool when it's just a buddy criminal movie with Michael and Lamar. But once it gets serious with the main story, he just seems like a third wheel.
Michael is half Frank and half Trevor which is probably why I like him the most.