Casters are the most powerful because of flare star (which lets you beat bozja duels in <30 seconds). RDM and BLM are both good, because RDM has self healing and BLM has infinite mana (which matters for flare star, which costs 9000 to use). It's pretty fun to walk around and kill everything on sight. Downside is it costs about 20k on my server to get or lucky drops from the zadnor endgame, but you only need one.

Healers and tanks are both fine to play, I like playing healers and there's lots of fun ways to boost your damage. Also, keeping your party (or self) alive through the FATEs is pretty fun.

One note: if you're prepared to spend a bit of money, getting resistance potions and essences which heal you as well as boost your damage make every job tanky.