1. Inspector Spacetime
2. Constant pop culture references.
3. They asked Batman eveything you've ever wanted to know.
4. The 8-bit episode
Digital Estate Planning is mostly an 8-bit video game.
I doubt this needs further explanation as to why it's as nerdy as awesome.
5. Remedial Chaos Theory
Remedial Chaos Theory is probably the episode that best represents Community's nerd appeal.
It's Troy and Abed's housewarming party.
You know, a normal affair.
But they roll a dice to decide who's going downstairs to get the pizzas they ordered
AND ALL HELL BREAKS LOOSE.
Yes, that's right.
THERE ARE DIFFERENT TIMELINES.
Plus, just like with Inspector Spacetime, there are glimpses of the other timelines in several episodes.
Not many comedy shows can pull this one off, and Community managed to do it in such an expendid manner, it gave birth to the next point.
6. Evil Abed
Having different timelines is pretty awesome, but Evil Abed breaks the charts. The Evil doppleganger is everything a villian should be, and watching him makes you wonder how "bad" you're doing in your darkest timeline.
I dare you to watch one episode featuring Evil Abed and not run to make yourself an evil goatee. I dare you.