Rather than trot out the same old faithfuls as everyone else, might I suggest broadening your horizons to some European fare? Obviously the Gothic series and The Witcher have already been mentioned, but there are a number of RPGs from Europe that have been translated into English, some of which are quite good. A couple that spring to mind that I have played are Drakensang and Arx Fatalis. If you like Gothic there's its spiritual successor Risen, which I haven't played but is apparently not bad.
Originally Posted by Sabretooth
I considered adding them, but from what I know, they aren't really essential games. Not having played them, I can't say much.
They're not essential certainly, but I'd say they warrant inclusion on an RPG list more so than Diablo (or any of its clones) does.