For me I watch American shows almost exclusively so it's
Heroes
A near perfect first season, hit badly by the writers strike the following year and just never recovered.
Lost
The final season. Invested so hard in this show and the last series is just hours of them wandering about in the jungle with torches to reveal what you suspected in series 1
*House Of Cards
*
So great when it started.
Obviously the production was put in a no win situation by Kevin Spacey, but even making allowances for that the last season may be one of the worst series of TV I've EVER seen. Total nonsense and an absolutely shit ending.
Game Of Thrones
I was a DIE HARD fan of this series, and not even I can defend HOW bad the final series was. They just threw away everything they spent years building story wise for shock value and spectacle. Unforgivable. It was obvious that the showrunners had tapped out, and they should've passed it on rather than run it in to the ground.
Anyone else?