In no particular order,
1) The Bard's Tale
2) Everquest
3) Dungeon Master
4) Ultima
5) Pool of Radiance
6) Baldur's Gate
7) Final Fantasy
8) Diablo
9) Wizardry
10) Daggerfall
Reasons:
1) Required everybody to get out their graph paper and make maps. One truely a big dungeon grind. First to use full color animated graphics that took up alot of screenspace in the day and age of simple sprites. It also provided 3d graphics dungeon crawling. Wow'd everybody of that time period. (1985) I downloaded the game last weekend to revive some of the old memories, I took this picture to share :
http://img401.imageshack.us/my.php?image=btld7.jpg
2) First MMORPG with 3d graphics which really caught on... First time people really got into the online game playing on a large scale. Sucked people in, forced parents to lock their kids in the closet, and for a good (or bad) reason.
3) The first quasi real time combat RPG with full color grahpics. Also had unique spell casting system which wasn't scroll/spell based, but rather "build your spell" using symbols, plants, and other mystical properties. Asheron's Call cloned this idea (before people whined it was too hard and they reverted back to scroll based), but this game was made in 1989 or so and influenced many games like Eye of the Beholder, Black Crypt, and those types of game.
4) Never really played it much, so I can't comment but I know it influenced alot of people. Richard Garriot AKA Lord British. I know the name and never even played any of his games. He musta done something right.
5) After Bards Tale and initial Ultima fame, this filled the nitch, had the official "AD&D" name. How many games came out using this engine/system? Too many to count.
(I'm getting bored of typing. I'll stop now. lol)