I suggest NIN, RPR or BLM. But pls notice that the playstyle of BLM, especially the AOE rotation, is changing nearly every 10 levels. It's not a big deal and not gamebreaking but you maybe look at a guide or cheat sheet for that. BLM is pretty fun and the more you level up it gets better and better. Best version is at 80+.

But to be honest nothing gets over RPR =)
It really depends on your preferences. Have you tried being a tank already?