I wanna know. For me, I would have to say "The Comeback Kid", it is just a perfect episode that utilises all the characters, shows so much about what the show is, has a great story, and shows us how much these people love and care for each other ("Work worth doing with people that you love." That quote seems to really illustrate what she was talking about there).
"Flu Season" is a close second.
Good news! Parks and Rec is going to be airing on TV again (as reruns). It's headed to Comedy Central where it will debut on January 21st with an all-day marathon.
As a Parks and Rec fan and as a California resident, this hit me in my heart strings.