Here are a few more that I love:
Kramer's polar bear friend who shows up at the intervention ("What...you don't want to be a polar bear anymore?")
The priest in the episode where George goes to his girlfriends aunts wake. The way he ominously says "I was about to get myself a snack" still cracks me up. But whenever I watch that episode with someone, they always wonder what I think is so funny.
The assistant costume designer from Spartacus who shows up at Kramers revival theater. Best line is when he finds Elaine's missing button: "My, what a spanking button."
-Jer