1. Favorite is Naga City for the epic feel. Unforgettable memories of the first run (wow 9 altars and I have 3 bosses to defeat before the final one, fun! But I die on first wave, not knowing what really awaits). And the second run, when I discover how epic it really is (wow a 2nd wave! It's okay I managed well! OMG A 3RD WAVE!!! OMFG I beat the final boss, thinking I had to kill 4 bosses, finally surviving 10!! *Throwing flowers at myself, feeling proud and achieved, opening a bottle of fine wine*).
2. Demonic Library. It first felt disappointingly easy (picking top tier classes at first, of course). But it has a great replay value because you can finish it with nearly (well The Avatar can just say: ) all classes, with varying gameplay styles. Fun mix of monsters and mini-bosses. (By the way, Naga City has a nice replay value too).
3. Dragon Isles. This "dungeon" was my first successful vicious run: I wanted to kill an uber dragon! No small feat indeed! The mana burn effect forces most classes to scum for the Soul Orb, which is annoying. Also,
all monsters there are extremely resilient. Not a funny mix.
4. Namtar's lair has a fun concept. But as I try to beat it with more classes, I realize that the first 2 forms are too strong against most classes, building frustrations (but that's probably just me, too lazy to think of a workaroud; I want to believe in devs when they say they want us to beat challenges with what we find as we explore, rather than with a pre determined strategy

).
5. Halls of Steel are harsh for most classes because there's almost only mean/vicious monsters and leveling can be quite difficult. I don't know why but unlike others I like having The Indomitable and Blablah in the same dungeon: one is best beat with many undiscovered tiles to regen fight, while the other is best beat with all tiles already revealed. Spawning The Indomitable not too far from the start could be a nice touch (making one of them deal physical damage could be nice too).
6. Gaan-Telet needs some work. It seems that the current implementation is to spot broken/too powerful classes and/or strategies. Tiles are a very scarce resource in there, so you have to use another kind of resource, and besides blood pools I don't see other options unless you achieve high resistances (with lifesteal to boot ?) and you're able to beat bandits without taking a hit.