In no particular order
Baldur's Gate series (400 + hour epic)
Icewind Dale series (Similar to above but with greater focus on combat)
Neverwinter Nights (Similar to Icewind Dale but smaller party size. Remembered mostly for its mod community)
Neverwinter Nights 2 (Better than the first in my mind for most of the game. Weak ending is compensated for in the brilliant expansion)
Morrowind (It's ugly. It's crude. It takes awhile to get lost in. But it is perhaps the greatest sandbox game ever made in spite of it all).
Oblivion (Same as above, but shinier and smaller in most respects. More polish and fidelity harmed the game in the opinion of many)
Fallout 3 (GOTY includes all the expansions. Similar to Oblivion)
Planescape Torment (Probably the best CRPG ever produced. It isn't pretty even with mods that make it a bit shinier but the game tells an incredible tale and the characters seem oddly alive considering most of them are dead).
The Witcher (The story, characters and character agency are stellar. Sadly, the game often resorts to silly fetch quests and useless grinding to fill time, especially in the early part of the game. Still, it features an active combat system that has a nice balance of player skill and character skill involved)
Diablo 2 (An action RPG. Click your mouse a billion times an hour. Collect loot. Somehow, this game consumed hundreds of hours of my life).
Gothic (Notable mostly for creating a world that actually seems like a world full of people rather than a world full of robots).
Dragon Age Origins (The best comparison is probably Neverwinter Nights 2 or Icewind Dale. Excellent voice acting and character work. Competent action and design all around).
Deus Ex (An FPS/RPG hybrid. I should warn you that if you didn't like the systems in Alpha Protocol you certainly won't care for them in Deus Ex)
Vampire the Masquerade: Bloodlines (Similar, in terms of systems, to Deus Ex. Incredible story. Excellent characters and voice acting. Community made patches resolve most of the rough patches).
System Shock 1 and 2. (the first might take some effort to run but it is worth the effort. The sequel is absolutely brilliant, if more than a bit ugly by modern standards).