The Star Trek franchise is probably the master of this. They had guest spots weekly and a lot of self-contained stories, so much that actor portrayed multiple characters over the years.
X-files had a ton of people, too. Giovanni Ribisi, Jack Black, Ryan Reynolds, Burt Reynolds, and a lot of others all appeared on X-files at some point.
In its peak, Laugh-In was a must-do cameo for celebs. Even Dick Nixon did a one liner: “Sock it to me?”
More X-files:
More X-files: Bryan Cranston, Tony Shaloub, Jane Lynch, Luke Wilson, Felicity Huffman, Peter Boyle, Charles Nelson Riley, Brad Dourif, Donal Logue, Dean Norris, Mark Sheppherd, Cary Elwes, Ed Asner, Adam Baldwin…
Not a TV show, but I’m amazed how many now-familiar faces showed up in Abbott and Costello movies
Ella Fitzgerald (In Ride 'em Cowboy (1942), her first screen appearance. And she sings, too)
Richard Carlson (In 1941’s Hold that Ghost, over a decade before he was big in SF films)
Bob Cummings and William Frawley in One Night in the Tropics (1940)
Shemp Howard (in Buck Privates and In the Navy and Hold that Ghost and It ain’t Hay)
Martha Raye in Keep them Flying
Sheldon Leonard in Hit the Ice
Arthur Treacher in In Society
Lock Martin (“Gort” in Day the Earth Stood Still; Martian in Invaders from Mars) in Lost in a Harem
Richard Deacon and Michael Ansara in Abbott and Costello Meet the Mummy
and lots of others – mostly because they were already famous – Margaret Hamilton, Boris Karloff (twice), Bela Lugosi, Lon Chaney Jr, Charles Laughton. And, of course, the Andrews Sisters and other musical acts.
Just about every celebrity has guest-starred on The Simpsons.
One of which was the classic Demon With a Glass Hand, and also starred Arlene Martel. Not a household name, but she had guest roles on every show back in the '60s and '70s. Probably best remembered as Spock’s wife, T’Pring, on Star Trek.