Say the first and second seasons (especially for high concept shows) before momentum and crashing so hard. The show could waste a great cast on meandering plot lines (partially because they likely already used up the really good ideas) that never even found resolution.

GLEE owns this thread.

Batman would be a prime example. It was a big hit in its first season but in the second the ratings would drop significantly. Originally airing twice a week, the show was cut back to once a week in the third season and added a new character, Batgirl. But it wasn't enough to save the show and it resulted in its cancellation in March of 1968.

Smash. I loved the first season, especially.

1. A Different World - Big time!
2. Mom
3. Mork & Mindy
4. Two Broke Girls
5. The Monkees

The OC

HAPPY DAYS

MORK & MINDY

Prison Break, however the recent revival made up for it.