Another vote for Arx Fatalis here - such a wonderful game, although the sword combat was shonky as hell. Magic was really satisfying, though. I think I must completed that game about 3 times.
Xenomorph on the Atari ST. A sci-fi dungeon crawl with primitive first-person shooting thanks to the enemies being able to move *within* each cell of the map (albeit in big jumps). Plus it had some very cool gadgets in it which really added to the atmosphere, like a motion tracker. Delightfully, despite being set hundreds of year in the future, mankind was apparently still using 3.5" floppy disks to store information.
And to choose one that ain't a first-person dungeon crawl: Divine Divinity. Just the most fun I've had in an RPG ever.