I like one cholo guy who has a high voice and always speaks slowly when fighting. I hate the Sharks because they are too powerful and if you ruin their business they riding on their Ranchers and attacks you. It's hard to survive there.
Jerry Martinez is my favourite antagonist in this game, closely followed by Marty J. Williams. I would have appreciated more missions from both of them. I never cared for the Mendez brothers at all. I feel that the Cholos, Bikers and Sharks came across as very generic villians. Couldn't we at least have seen their bosses? I mean, you even wipe out the Cholos in Umberto's last mission but we never get a boss fight. And what about the Trailer Park Mafia? They fight you in a few storyline missions and disappear forever when you kill Marty. IMO they should have been used as a rival gang in the empire site war. I liked the Shark death squads though. I consider myself to be a really good GTA player and even I have to look out for them.
Most liked antagonist: Jerry Martinez. He creates a lot of tense during the Storyline, so you never get tired of him.
Most hated antagonist: Marty J. Williams. This is just one dumb prick that I had pleasure killing. Although some of his missions are interesting ("Waking Up The Neighbours", "O Brothel, Where Art Thou"), he acted like a big shot although he wasn't.