I daresay that the TV version was the best rendition of Neil Simon's story, which originated as a hit Broadway musical starring Walter Matthau and Art Carney. It was made into a popular movie starring Matthau again, this time alongside the brilliant Jack Lemmon. Yet, nothing can top the comic genius performances of Jack Klugman and Tony Randall.
Others have played the roles on stage, including Pat Hingle, Eddie Bracken, Paul Dooley, Martin Short, Eugene Levy, Pat Sajak, John Larroquette, Matthew Broderick, Nathan Lane, and Brad Garrett. The show has been performed countless times over the decades -- I myself played Speed in a university production.
The award-winned play was updated as Oscar and Felix: A New Look at The Odd Couple, and the character even switched genders in The Female Odd Couple with Sally Struthers and Rita Mareno.
TV producers tried to recapture the magic of the Klugman/Randall series with a reboot called The New Odd Couple starring Ron Glass and Demond Wilson. Another new version starring Matthew Perry is in the works as a mid-season replacement for CBS. There was even a cat-and-dog cartoon called The Oddball Couple.
It's a great concept, which explains its longevity, but the TV show itself was what put it in the stratosphere of comedy classics.