I have to go with 'I like them all.'
Each were so well crafted and really sold their personalities.
Michael seems to be the least favorite...but the way he consistently seemed on the verge of mental breakdown due to all of the craziness in his life...and when he lets it loose..it's golden.
Trevor is the fan favorite, no doubt. You sometimes feel he's just a normal guy, and sometimes feel like he's the most vile person alive. You consistently struggle to choose one or the other, and in the end, you get no answer. And that, is why he's a great character.
Franklin is just a guy trying to make it big. The other two have sometime to live up to or to make up for internally, but Franklin? He's just in it for the paper, and amongst the many squabbles there is throughout the monologue of the game, Franklin is always the one there straightening them out and keeping our 'heroes' compact.
Together, they make a fantastic trio and some of the best characters I've personally seen in gaming.