Take the time to sit down with a nice brandy and reminisce your trials and tribulations in the dungeons.
Fun:
Shifting passages hard - open, enjoyable and let's me blast the crap out of walls. The shrinking effect is very fun.
Halls of steel hard - I found it difficult with limited black space and creature abilities. Made it harder to power level. It improved my play style the most while being challenging with different classes and races.
Demonic Library vicious - I find I have to pause and think about what to attack and take my time exploring but the fun is with the avatar. Nothing like having more monsters to bash and see how many levels I can gain.
Most annoying:
Labyrinth hard - I spent so much time on labyrinth with different classes getting smashed until I worked out what's strong. And how the boss moves.
Ick swamp hard - picking monk for this level annoys me. I first tried the monk here. Lol worst decision.
Naga city vicious - frustrated the hell out of me before Avatar told me how to deal with it. Like most dungeons the first time is the hardest. I struggled so hard with gold and what god/s to pick.