Generally I appreciate RPGs that allow me to specialize my character with different stats, and have a great replay value. Oddly enough my favourites (the Gothics) sort of violate this, but they have other redeeming points
In order of enjoyment
Gothic 1 (for the story, world, and NPCS)
Gothic 2 with addon
Morrowind (for its replay value)
Baldurs Gate 2 with addon (for the NPCs and replay value)
BG1
Realms of Arkania Blade of Destiny (for the cool setting and ridiculously elaborate skill system)
Darklands (for a "realistic" environment and a neat skill system)
Icewind Dale (for multiplayer hack n slash, its not much of an RPG)
IWD 2 (as IWD1)
The Goldbox games of old, particularly the Krynn trilogy (for nostalgia, but they are still fairly playable, its just unfortunate that non-humans suck in most of them).
Also played Might and Magic III, VI, VII, and VIII quite extensively, but they are only really fun for developing characters and skills. Pretty crappy RPGS. Never got into the Fallouts or Arcanum as I couldnt take the combat system (which saddens me as especially Arcanum has a very cool setting) and the weird levelling of party members. Oblivion wasnt my cup of tea at all. Some old SSI games (Darksun?) were fun-ish but had poor interfaces. Eye of the beholder aged pretty badly (and this is from an old school gamer) but was fun for its time. Darklands also aged pretty badly even if it was superb back in -96.
It's sad that there are so few RPGs that a top 10 list covers half of the ones I've played