I like any of the 1st person-oriented games. Currently, I've gotten the most mileage out of Wizards and Warriors. Before that, it was Daggerfall, then Arena, and most of the Wizardry series. I also really enjoyed the Might and Magic games (although I really disliked the sci-fi aspect of the later ones. NWC has caught a lot of flack for that recently).
My overall favorite: Hands down, it's Wizardry 7: Crusaders of the Dark Savant. Reaaaally looking forward to Wizardry 8! (I'd better: I'm the moderator of that forum!

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