For the ARPG nut:
Titan Quest is great, it's a lot like Diablo 2 with some nicer features and a freaking AWESOME class system.
Sacred Gold is good as well, it's a giant level scaling world populated by about a million things to kill, including giant spiders the size of a large SUV and dragons that dwarf those. The classes are cool, and the story is a little better than the normal ARPG "OMG Big bad evil, go kill it."
Torchlight 2 is .. basically Diablo 2 Redux. Good for some time killing but just.. lacks the staying power of Diablo 2 (for me at least)
For the 'Oldschool' RPG fan:
Freedom Force (and it's sequel) are both very fun games.. think Fallout with super heroes.
I confess I've not played the Avernum games, but I have heard great things about them, and I do enjoy "Avadon, the Black Tower" from the same company.
Planescape: Torment is not my cup of tea, never was able to get in to the story and the gameplay didn't hold my interest. That said, it's got one hell of a story and some great characters- well worth giving a shot at very least.
Baldur's Gate series: I've only played 1 and I frankly hate it. I don't like the cliched start and I detest D&D 2E for video games- it's NOT meant for video games and it makes crappy rules. If you can get past the cliches and the crappy rules and the annoying start... it's worth picking up.
Ice Wind Dale series has 2 redeeming qualities (to me) 1) build your own party. 2) the 2nd uses 3rd edition rules which are build for combat.. and I'm a 3rd ed DM so I know the rules inside and out. That being said, story wise they're not as good as the BG games.
Temple of Elemental Evil, if you can get past the Troika bugs is a great hack and slash "Oldschool" RPG. It's highly combat focused, has jack squat for a plot, but has D&D 3.5 mechanics and the ability to build your own awesome party with some epic synergies.
For the Western RPG fan, I'm not much of a fan of this genre myself.. seldom do I get immersed in them...
Obviously Oblivion.. I think it's a bloody awesome game, it's the first TES game I've played that I REALLY got in to vanilla.
Vampire the Masquerade: Bloodlines.. Once again, Troika bugs.. once you get past those though you get a pretty nifty RPG, fairly linear though.. makes up for it with some cool characters and being a non-sparkly Vampire.
Fable is an OK game, but I have one major problem with it: you age as you get XP. You, and everyone around you, can age like... 60 years in just a few months. Also your appearance is determined by being good or evil., Immersion breaking abounds.