Some instances are combination of themes, so this isn't exactly asking what your favorite one is. Though I'm sure people's opinions will be influenced by which dungeons they have the most fond memories of. The question is asking which types of enemies you find it most appealing to take down.

evil dragons, it has so much magic in it

Evil Dragons, its REALLY tough to beat the bad ass factor of storming a castle to take out an evil ancient dragonlord.

Second would be Evil Orcs, i LOVE BC dungeons. Nothing for me will ever beat running dungeons like ramps, in fact i still find Ramps to be the best most balanced dungeon ever released on WoW.

the scourge is the best looking/well made for me

but my fav is dungeons like Zul Aman and Zul Gurub