My top 3:
The Marine Biologist -
Kramer takes up golfing by the ocean, and ultimately ends up fine-tuning his accuracy. While George's (a.k.a. Art Vandalay) wish was to be an architect, he is cast as a marine biologist and must play the part when a beached whale needs help. In typical Seinfeldian fashion, the final twist leaves you gasping for air... a hole in one indeed.
The Contest -
AKA: The Master of Your Domain episode. Upon being caught gratifying himself by his mother, George decides to cease these practices and enters a contest with his friends. As expected, the contest becomes more challenging as each one is confronted with a rather tempting situation, making it more difficult to remain master of their domain. Undoubtedly, one of the most talked about episodes ever.
The Hamptons -
Everyone makes a trip to the Hamptons to "see the baby" and enjoy the weekend. George's girlfriend goes topless and everyone sees her chest before he does. "The baby" is the most hideous thing they've ever seen. Elaine is confused when a handsome doctor refers to her as "breathtaking" - the same term he later uses to describe the baby. Kramer finds a filled lobster trap. Jerry's girlfriend sees George naked, after he's just gotten out of the pool, so something isn't as it should be. Shrinkage!!