Temple of Chaos is a bugger in the original/origins version, but like most things in DoS it's a bit of a joke. Pandemonium will rip your eyeballs out unless you're highly trained (like me, who refuses to go in without 999 MP, 7000+ HP and 16 in relevant weapon/spell skills). Dark World again depends on how much you've trained, but it ain't too bad. Lunar Dungeon can be hard also especially if you try to take out all the sub-bosses as well. N-Zone is more fun than hard, unless you count Omega Weapon and Shinryu. Kefka's Tower can be easy if you have the right kind of parties. The Northern Crater I can complete with my eyes closed. Same with Ultimecia's place. Memoria I remember actually being alright, not too hard or easy, but I had trained extensively prior. FF10's last dungeon is an utter joke (though really the only decent dungeon in that game is the Omega Ruins). Never played FF10-2, so I can't comment, and no one knows what FF12's is like yet.

From this I guess I'll choose Pandemonium.