Quote Originally Posted by Behold the Void
I'd say Final Fantasy Tactics.

Especially because of this:

I actually went back and leveled up so much for that fight after getting my butt kicked a few times that I never had to spend time deliberately leveling up for the rest of the game, and it was still a bit of a tough fight. Aside from that though, Tactics wasn't that hard a game for the most part.

I'd probably give it to one of the NES FF's. I've barely played any of them really, but I have played enough to know they're hard as hell (at least early on), and it takes forever to level up in the first one.