If this is simply about the worst game, then I'd say Outpost.
But if this is about games you initially have high expectation, then I would say the following games:
WarCraft III
Despite its healthy, vibrant colors, its excellent storyline, its role-playing elements, its easy-to-use menu (which makes it easy to micromanage), and its innovative elements like auto-casted spells, WarCraft III has the most disappointing gameplay compared to StarCraft or Red Alert 2 (the games I played prior to playing WarCraft III).
The problem is: I just cannot rationally, logically tell what's wrong with this game. I just feel that the game is boring, where each mission feels nothing but obligatory chore to go through its excellent storyline. This is much different than Red Alert 2, where my reason to go to the next mission is because I can't wait to harass my opponents with terror drones again (and Kirov, and Dreadnought) instead of discovering the next cutscene.
Dungeon Siege
This is also a game with great potential, but turned out to be linear and boring. Make no mistake, I love Dungeon Siege's "free-form" character advancement; the lack of character classes, where we shape the skills and abilities of our character by our gaming actions (just like in Quest of Glory) instead of having to advance through levels. I would also love the fact that we can have NPCs that are smart enough to fight by themselves, where we just have to set formation and tactics, and the AI will take care of the rest. It should be noted that Ultima VII is still disappointing in this matter.
But.... whole gaming experience is just too boring. The game world is sparse and empty, and while we have the freedom to go back and forth between locations, there is no real incentives to do so, because the previously visited locations are already empty. Now, respawning enemies is a pain in the ass in first-person shooter, but non-respawning enemies is a source of boredom in role-playing games.
Mech Commander
Oh, the entire premise sounds promising, and the gameplay is actually enjoyable. The problem is: the game is just so damn difficult! A friend of mine said that IL-2 Sturmovik is the most difficult game ever, but at least I can still handle its legendarily punishing flight model . Mech Commander, on the other hand, makes me give up in the second mission. Maybe I'll try it again when I have more time, or maybe not.