Zandronum

and it was the one where Kramer got new car plates but it was not his... It turns out that his plates has "ASSMAN" on it. So forcing on the Kramer storyline, he rolls around the city(in one scene) as Dr.Cosmo Kramar, the Proctologist. Even got a note from a thick girl who wanted to date him because he's known as the ass man of New York. AS the show ended it turns out that person who order the ASSMAN plates were really from a Proctologist. Which was a real joker at the end.

In your opinion, what's a good seinfeld joke?

I think you will like this blog: http://seinfelt.tumblr.com/
It has simulated plot synopsis, if the show was running to this day.
Personally, I was watching show as a kid and all I remember was Soup Hitler.

I find it really hard to fully appreciate anything from season eight or nine, because Larry David was not around for writing. Those who enjoy early Seinfeld must seek out "scriptless" show Curb Your Enthusiasm

Of course, the amount of good seinfeld moments are countless but one comes to mind
http://www.youtube.com/watch?v=yKlxy1ZFhMA

S2 is great but Season 3 has to be my favorite.

Edit:

Theres more to that assman bit, that very proctologist is assigned to work on george's father, who get's Kramer's noodle statue stuck up his ass. The proctologist mentions his patients all say the same thing - which is the line that ends the episode:
"It was a million to one shot, doc. Million to one"
-Frank Costanza

Well I'm an ass...
yes, I'm a ass man...