For me:
The old classic is of course: Lands of Lore I
It really showed off the capabilities of CD-ROM when it first came out, (the intro was showing at our local games store) and it actually persuaded my parents to upgrade our PC to have 16 meg of RAM and a 4x CD DRIVE so they could play it.
Of course, at the time, being a young lad, I was more interested in Doom and Dune II. Oh, and Tie Fighter.
I never saw the PC version of Dungeon Master, but I did play it on Atari ST. Again, at the time, I was more interested in completing Millenium 2.2, but I do remember trying to map Dungeon Master with Graph Paper. My map was utterly terrible and I got killed, so I never played again.
In actual fact, my love for Dungeon Crawlers is a more recent thing, awakened by playing the rather awesome Etrian Odyssey on the Nintendo DS. That is a truly great series; being able to draw your map on the touch screen is a great idea; and its a neat fusion of western and eastern RPG concepts, with the 3d grid based movement mixed with turn based battles. Not to mention its cruel. Real Cruel. FOE!!