I'm not really sure how to respond to this topic, since I can usually find at least one redeeming quality in any game. So I guess I'd consider the 'worst' games I've played to actually be the 'most disappointing' games instead.
Any Command & Conquer title from Generals onward. Command & Conquer: Generals was C&C in title and in title only. It had no connection to the rest of the series in terms of story or gameplay (beyond being and RTS). It's not a bad game, but it's not a Command & Conquer game either. I probably wouldn't have been so disappointed with it if they'd just dropped C&C from the title. As for the rest of the games, which actually do connect with the rest of the C&C franchise... I've just been very disappointed with the quality of them. They're not terrible games, but they just don't feel very C&C to me. Command & Conquer 3 came pretty close... but still felt... off. I couldn't even bring myself to finish Command & Conquer: Red Alert 3... which makes it a bit of a rarity in my game library since there are very few games that I don't even bother to finish.
Then there's Deus Ex: Invisible War. Not a terrible game, but like Command & Conquer: Generals... it really should have just dropped the Deus Ex part. It lacked pretty much everything that made the first Deus Ex my favorite game of all time, and whatever wasn't removed ended up being "streamlined" (IE: watered down) to make the game more... accessible. I've played through Deus Ex probably 15+ times... I've played through Deus Ex: Invisible War exactly once. And I have no desire whatsoever to increase that number. It makes me extremely sad.
Probably gonna get some hate for this last one... but... Bioshock. Again, not really a game I would call "bad" per se... but definitely disapointing as someone that's a huge, huge fan of System Shock and System Shock 2. I wasn't too upset that they were going with a new IP instead of making a System Shock 3... I'm okay with that. What disappointed me was that, in my opinion, everything that Bioshock tried to do... System Shock 2 had already done it better... eight years earlier. I played through Bioshock with an open mind, and actually did enjoy quite a few things about it (the twist was great - in my mind it was on-par with the big reveal in SS2)... but I couldn't help but feel like I was playing a... "streamlined"... version of System Shock 2, which made me want to just go back and play SS2 again instead.