Baldur's Gate I & II are superb, still play BGII to this day. 300+ hours of gameplay for a single play-through if you want it. Very dynamic, some fights are very hard but incredibly rewarding. A must for any real PC gamer & RPG fan.
Mass Effect was brilliant, played it through fully (40-60 hours) 6 or 7 times between the PC and 360. BioWare haven't let me down yet.
KotOR is fantastic. Brings the D&D ruleset into a very easily accessible game with a gripping story & characters to boot.
I'm hearing good things about the Restored Content Patch [http://www.deadlystream.com/showthread.php?t=339] for KotOR II, only recently played through it again so will be putting this one off for a while. (A lot of content was cut from the game to get it out for Christmas - including a whole planet!)
Gothic III with the Community Patch [http://forum.worldofplayers.de/forum/showthread.php?t=683897] that was released last week isn't too shabby, at least it's playable now!
Haven't had a chance to get Risen yeah, but the demo seemed promising.
If you haven't played Diablo II yet, it can be had for £10 digital download (including expansion) same goes for Titan Quest Gold
I'm also about to try Pahncrd's Oblivion Enhanced Edition RC1 [http://www.rage3d.com/board/showthread.php?t=33951436] a ~12gb modification for Oblivion, alters pretty much everything in game for the better, but only played 2-3 hours of the beta.
Wasn't a fan of Fallout 3 on release but have pre-ordered the GOTY edition with all the DLC to give it another chance.
Must haves;
Baldur's Gate II + Expansion
Knights of the Old Republic
Diablo II + Expansion
Oblivion GOTY Edition
Mass Effect
Maybes;
Risen
Gothic I / II / III
Fallout I / II / III (GOTY Edition)
Baldur's Gate I + Expansion (Modded to play on BGII engine)
Titan Quest Gold
Knights of the Old Republic II
Keep an eye on;
Mass Effect II (BioWare)
Dragon Age: Origins (BioWare)
Borderlands (Gearbox)
Torchlight (Made by co-designers of Diablo and Diablo II)
As a side note - JRPGs should have died long ago, their gameplay was a product of technical limitations and is nothing but homo-erotic chess.