Be it a sequel that was worse than the original, or a hyped up game that was pants.
1. Driv3r - A total mess of a game that was hyped up to the 9's, possibly the most broken game ever.
2. Killzone - Not truly awful, but so hyped up it should of been so so much better.
3. Vandal hearts 2 - The original was a cracking game with loads of charm, the second was a souless mess that had little connection to the original and fucked the gameplay up.
4. Suikoden 4 - The original was flawed but fun, the second was a fantastic piece of work, and the third is apparantly supposed to be one of the best RPG'S ever....the fourth was a dull and boring game, which was 90% set in featureless sea, and managed to make pirates boring.
5. Final fantasy 8 - Not a bad game as such, theres alot of ff8 I love, but the awful magic system, a shite plot, and an unlikeable main character mean't it wasn't a patch on the masterpiece before it.
Special mentions:
Broken sword 3
GTA san andreas
oddworld abes exoddus
Wipeout 3
devil may cry 2
enter the matrix
Jak 2
True crume