morrowind + 2 expansions is definetly the way to go for an open-ended rpg. you could get so carried away with the "side" quests that you could play the game for days and never hit the actual plot.
wizardry 8 was pretty open ended as well, but not much to do to really warrant moving off the main story line.
the baldur's gates/icewind dales are still a little too story driven since you'd just get killed trying to go to any higher levels, but they have their little side quests along the way.
in the end, morrowind is the best choice, but you'll need a fairly powerful PC seeing as how the engine isn't too efficient.
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