A major reason why the show works so well is the ease in which the cast plays off each other. The jokes are well-written, of course, (especially when compared to network TV shit like 2 Broke Girls), but there always seems to be lines that had to be improvised. Season five's gag reel, collected in five parts below, isn't only just hilarious—it shows that those great interactions really well.
(Also, Perd Hapley is the best obscure character in TV history. Just need that on the record.)
[H/T: Complex's Angel Diaz]