Bit of a tie:
Pool of Radiance: Ruins of Myth Drannor. Boring
and reformatted my hard drive.

Temple of Elemental Evil. Great concept and combat mechanics, and some lovely looking areas - but buggy as all hell, some dismal writing and really uninspired encounters. Shame. I hate that this is on my list of worst games, cos it had so much potential.
Deus Ex: Invisible War. The Dragon Age 2 of its day in many ways. In itself not the worst game, but to many people (including myself) an awfully disappointing sequel to an amazing game. Which is why it's on this list.
Oblivion (vanilla). Half-arsed main storyline, repetitive looking dungeons/forts/cave systems/Oblivion gates, poor voice acting, some of the least memorable NPCs in any game, clunky combat, very bland and generic atmosphere, generally bad writing and far, far fewer cool things going on than in the very enjoyable Morrowind (eg flying/levitating, teleporting, silt striders, Dwemer artifacts, Daedric ruins, the breathtaking number of factions you could join). It had a few saving graces - a handful of decent sidequests (sadly mostly the Brotherhood ones, so only evil characters got to experience them), very pretty for the time (though the exploration got old a lot quicker for me than in other Bethesda titles), Shivering Isles was apparently pretty good and there are some mods which improve the game a fair bit. But still nowhere near as good as Daggerfall or Morrowind. IMO, obviously.
Sorry, that turned into a rant.
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