Super Monkey Ball - The first two on Gamecube were amazing. All of the sequels after that have been medicore at best.
Castlevania - I loved the old school classic Castlevania games. Once the series changed to the Metroidvania formula, it lost me.
WWE games - Wrestling games used to always be fun and something that I looked forward to every year, but ever since 2K took over the license, the games have been garbage.