Rama's real nice for PvP if you're into it, and Shisa, which I imagine you've already finished, is his best dungeon job, more or less. So I'd say Jin as well. Leveling him to 40 should be cake now. I got my Jin from 31 to 35 in an evening with the new missions.
Sieg's 4th is going to be awesome, but it will still be several weeks before it's released for us. Ronan's 4th is also nice, and his 3rd is currently bugged and doesn't require PvP, so if you're worried about getting 3rd done, he'd be the one to take.