Dexter jumped the shark almost immediately in the second season. The show had such promise but then it just kind of went bland. It lasted way to long and stopped being watchable way to early.
I guess 90210 by the fifth season it was alright but not that good.
The Simpsons without a doubt. It should have ended in 2000 in my opinion. If it did, I think it would have cemented the status of being one of the most consistently awesome sitcoms of all time.
I love Family Guy, I grew up with that show but it started to really suck after like season 7.