To elaborate: Agent 47 is there to blend in cool as crystal and ice his target. Doomguy is there to rip and tear demons until it is done. Trevor is there to fuck, fight, steal, and wreak havoc. These characters aren't just protagonists or player avatars, but vessels for their game's goals. There are no conflicts within them that impact the narrative anti-climatically.
Trevor has conviction in doing the main things you do in GTA. That's why he's the best.
I don't know really. I don't remember any of them particularly well, except for Trevor - but that's because he was so over the top. I suppose I enjoyed his portrayal the most, but I do have a thing for that sort of character type.