Tie between S1 and S4 for me.
S1 introduced the humor, which I wasn't expecting at all when I initially saw the series, so I loved that. I still find all the old banter between Buffy, Xander and Giles to be gold. And Willow back when she was still Willow...
S4 is hysterical, but only because of Spike. In a very, very lackluster season, he saves it. And "Pangs" is one of the most low-key hilarious episodes of the series.