There are a lot, but this one comes to mind:

"Why are you the way that you are? Honestly, every time I try to do something fun or exciting, you make it not... that way. I hate so much about the things that you choose to be." -2.22 Casino Night

I love "That's what she said"

But let me write another one :P
"Fax ? Why don't you just send it over on a dinosaur? " season 5

"You know what they say- Fool me once, strike one. Fool me twice, strike three."
-Micheal Scott

Best quote ever from MS : link

"Nooooooo God Nooo" ... "Noooo" (again)

Go follow the link on youtube multiplier to enjoy this epic "No". Or maybe there are a lot of "No" :°

OOH oOh if I had hitler and (someone else) and Toby in a room with two bullets and a gun I'd shoot Toby twice