Regardless if they've been said or not, here are the game I and most others think are worse than the first (some of which are still good enough games):
Mercenaries 2
Driver 3
Deus Ex: Invisible War
Perfect Dark Zero
Fighting Force 2
Devil May Cry 2
KOTOR II
Red Alert 3
Supreme Commander 2
Empire Earth III
Age of Empires III
Silent Hill 3 - 5
Sonic... anything
And of course anything from the early PlayStation lineup like Spyro, Crash, Tomb Raider, Tony Hawk etc that are no longer popular.
But in general sequels are usually better for video games.
Mercenaries 2
Driver 3
Deus Ex: Invisible War
Perfect Dark Zero
Fighting Force 2
Devil May Cry 2
KOTOR II
Red Alert 3
Supreme Commander 2
Empire Earth III
Age of Empires III
Silent Hill 3 - 5
Sonic... anything
And of course anything from the early PlayStation lineup like Spyro, Crash, Tomb Raider, Tony Hawk etc that are no longer popular.
But in general sequels are usually better for video games.