http://www.jumptheshark.com Is a great site! You can go there and vote on television shows' moment of decline. The name came from Happy Days, BTW.
So, when did you realize your favorite show (or least favorite show) jump the shark?
FAV: I said "Vengeance Unlimited" never had a chance to jump the shark, because they pulled the plug too quickly.
I can't limit my least fav to one.
Other: I said "Friends" jumped the shark when Ross and Rachel got together, and really hit the dirt when Monica and Chandler got together.