Whenever I'm in the mood to play a dungeon crawl, I keep coming back to Four Against Darkness: it's the best system that I've come across for this. Its rules are elegantly simple & modular: very streamlined & fast to play solo or GM-less; as well as easy to customize, absorbing to tinker with. Yet it feels similar to playing traditional dungeon crawls & classes from the 1970's: it isn't as clunky but it scratches that itch. Plus its dungeon generator is a lot of fun to draw maps with. I love it.