I'll just assume that you refer to "old-school" first-person dungeon crawl games, but this is a question I've tried to answer for a long time. I've played quite a few of these, and have yet to find one that has everything I want.
The Might and Magic series is pretty good. I particularly like M&M 2 just because it still had a lot of old-school flavor, but still had a pretty big world to explore and lots of stuff to see and do. M&M 6-7 are good too other than the poor race/class selection (and the horrible portraits).
I actually don't like Wizardry and Ultima that much because of the Sci-fi meets fantasy thing. That's a little too weird. The old gold box games bugged me because of the Dragonlance setting (Can't stand Dragonlance), but they had some pretty cool mechanics. If I remember right, there was actually a Gold box builder program where you could create your own game.
Another game that I really like, and often gets overlooked is Dark Heart of Ukruul
. This is a pretty tight, polished game (for old-school) that is pretty cool.
Another game that I feel is pretty underrated is Wizards and Warriors
. Not sure why this game got such a poor reception, but I thought it was pretty fun, and had a very unique movement engine.
Other than that, I'm still looking for one that would combine a huge world, with a generic (non-IP) setting, no sci-fi stuff, light on story (more free-roaming), and plenty of randomly generated content. I haven't found one yet that does all of that very well.