I've got so many of these... By the way, you're probably all gonna yell at me for being so epicly controversial, but I honestly don't care! =D
1) System Shock 2:
I'd just like to clarify to all those who are now thinking "Duh, SS2 didn't have a sequel" or "WTF IS SS2?". I'm counting Bioshock as the sequel because; well; it essentially was the same game, except shit.
2) Halo 2:
Halo 3 was just pure fail.
3) Modern Warfare:
This one doesn't even need to be explained.
4) Fallout 2:
Well; I'm sorry, I don't wish this didn't have a sequel, I just wish the sequel was actually good, rather than just an overrated interactive movie... At least New Vegas will have aim down sights for people who actually want to play a game.
5) Deus Ex:
Again; doesn't need explaining.
6) Metal Gear Solid:
Nothing more needed to be done; yet it was anyway...
7) Final Fantasy 7:
The series has gotten worse and worse and worse and worse and worse and worse and worse and so on and so on and... Ok I'll stop.
8) Painkiller:
It was just more of the same; except worse.
9) Age of Empires 2:
So many that don't need explaining...
That's all I can think of right now.