Way too many to choose from - Puddy is always hilarious and Bookman is one of my favorite guest spots, and I love when George is distraught because women don't know about 'shrinkage' (Elaine's reaction "It shrinks?!" is priceless). Puddy and Elaine's airplane trip is gut-busting - but I think if I had to name one episode that kills me every time I watch it, it has to be the Merv Griffin set Kramer recovers & George's injured squirrel episode (when the vet tells George he can visit the recuperating squirrel and tells him "you have half-an-hour" and George looks at him like 'what the hell am I supposed to do with an injured squirrel for a half-an-hour?!". And when Kramer breaks for a snack and then announces "Ok, we're back").
Oh, but wait, I just recalled - my favorite has to be the bris/pig-man episode . . . brilliant.