Ditto for Morrowind, although it's sequel ("Oblivion")should be out soon, and it looks to die for.
Both of these games are limited to single player, though.
There's a big following for Neverwinter nights, which is great if you're a D&D fan and like the idea that's it's capable of allowing human dungeonmasters to control a multiplayer server in realtime, adding monsters and speaking for NPCs.
There are some decent single player modules for it too. The graphic engine is a bit dated, still looks OK but nothing special.
There's a sequel coming out for that too, but won't be here for a while, at least I haven't heard any news about it lately.
Also deserving of honorable mention is Knights of the Old Republic, one of the best narratives in an RPG ever, but not much replayability there.
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