Hullbreaker Isle is my new fave. I don't even mind having to farm it over and over because the changes in location are fun. You start out on a beach (I like luring mobs into the bear traps), then you beat up Donkey Kong, then you do a little Indiana Jones spelunking with the tablet doors, then some more beach fighting, and then a unique boss! I like how the sub-bosses have fun mechanics and how the main boss fight covers an entire set of mini-islands. It's way more refreshing of a variety than the usual dungeon.