I'm going to cheat technically it's a raid but Syrcus Tower I used to really enjoy when that was a big thing. The music and visuals are beautiful and it wasn't particularly difficult for most people apart from the first boss when one party seems to get all the ice for some reason and no other healers have the mind to support the party with two frozen healers but I loved doing that one.

Others I've enjoyed over time Cutter's Cry, Keeper of the Lake (Even though everyone seems to hate it for some reason), Qarn HM, Fractal Continuum, The Vault, Wanderers Palace both normal and hard mode.