Some games were just so perfect when they were first released, that a sequel is not only superflous, but downright detrimental to the original. What games do you wish never received sequel(s)?
For me, No More Heroes and Pikmin are such games.
NMH2 was a mess. It had no internal logic, which made the game even more (and IMO too) hedonistic. That and the bosses were no fun.
Pikmin on the other hand was just perfect. It's not like 2 and 3 ruined the franchises, but they expanded upon the original's very focused core concept and some of the magic was lost on the way.