NBC (ended 1998)
By far, the worst episode in my opinion is The Deal. I have never wanted an episode of Seinfeld to end more than that one. It actually hurt to watch it.
Others I don't like are:
The Opera (only seen it a couple of times, and never found it that interesting)
The Barber (the whole storyline really bothered me for some reason)
The Race (another episode I just have never gotten into)
The Understudy (I always thought this episode was way too overhyped)
I'm sure there are others, but I haven't seen seasons three or nine in quite a long time.
I can't believe everybody liked "The Limo". That and "The Dog" were by far the worst episodes of the entire series!
I loved "The Dinner Party". I can't believe somebody said they hated "The Mango"! I was completely shocked!
"The Deal", in mine and many critical opinions, has one of the funniest scenes in television history. The opening scene with Jerry and Elaine is pure comic genius.
Seasons 7-9 touched on the absurd, but none-the-less fantastic.
I must say though, I really don't like "The Caddy" very much. Besides Elaine's storyline, that episode really fell short with me.