I would like to propose that the episode in which George is forced to abstain from sex b/c of his girlfriends mono and becomes intelligent all of the sudden is one of the best Seinfelds ever. Sure there are other really good ones like the episode where Jerry lives in Kramer's apt. b/c of the chicken place (another one of my favorites) and many more that are just classics. However the abstinence episode is "classic Seinfeld." In addition to George's sudden intellect it also has Elaines dating the doctor and Kramer's apt. changed to smoker's lounge. The part where Jerry asks Kramer why he is inviting all of the smoker's up to his apt is classic.
Kramer: "Well, you can confine them, you can throw them out on the street, but their not going away Jerry!" (something like that)
Jerry: "all right"
Kramer: "Yahh Yah"