There is no TV show with more connections to the New York Mets than Seinfeld. Sure, George Costanza may have worked for the Yankees, but that didn't stop co-creator Jerry Seinfeld from letting his Mets fandom out. Whether it's Jerry taping the Mets game during the pilot, or the Mets hat in Jerry's apartment, there is no shortage of Mets cameos. You just have to pay attention during the episodes.
However, the most iconic moment of the Mets on Seinfeld is when Keith Hernandez guest-starred in the two-part episode "The Boyfriend" during season 3.