I find comedies the easiest to re-watch. My top ones would probably be:
1. Scrubs (I have probably seen every episode 3+ times)
2. Psych
3.How I Met Your Mother (mostly earlier episodes)
4. Eureka
5. Chuck, although the last season depresses me too much so I usually can't watch it.
And for non comedies:
1. Game of Thrones
2. The Walking Dead
3. How It's Made. For some reason watching factories at work never gets old.