Mass Effect of course, from game 2 on,
Ratchet and Clank, which has finally managed to kill itself forever in my eyes.
Jak and Daxter, which got entirely retarded during Jak X, and got WORSE with Lost Frontier
Kingdom Hearts: afflicted with FFVII's own problem, though not as severely. Some hope still remains.
FFVII: everything released in this series after the first game has been progressively shittier.
SSX: Blur was a misstep in every way, and SSX 2012 was good, but had several new problems, most seriously the lack of real multiplayer and the awful shop system
Assassin's Creed: 2 made the game less "boring", but starting from then on, the games became more stupid as well, culminating in 3 which is a failure in nearly every venture
The Elder Scrolls: hasn't lost its way exactly, but definitely needs to bring back some older design choices, and the dedication to story seen in older games.
There are more, but I think that's enough bitching for now