Best guest star?

Lori Laughlin

Jami Gertz for not having a square to spare.

James Spader is one of mine. Called out George time after time for stretching out the sweater.

I think in terms of recurring guest stars it has to be Newman. You werent so specific. But others include a lot of others. But yeah I think it has to be Newman.

Marisa Tomei

The people who replied to this thread don't seem to know what a "guest star" is. Keith Hernandez, as mentioned by the OP, was indeed a guest star. Bette Midler and Raquel Welch were also guest stars. Marisa Tomei's and Jon Voight's appearances were appearances by a star, but were more of a cameo than a guest star role.

I can't think of any other guest stars on Seinfeld, except maybe for Teri Hatcher, but only in her second appearance (s09e22 - "The Finale"), because by that time she had starred in her own TV series. In her first appearance, she wasn't yet a star of any kind, because it was about six months before her breakout role in Lois & Clark: The New Adventures of Superman. But her second appearance was also more of a cameo than a guest star role. Plus there was her pre-stardom appearance as the same character, so that muddies the waters too.

The early Star Trek trifecta of Gene Dynarski, Clint Howard, and Phil Morris.