M*A*S*H was all over the Pyramids: Jamie Farr, Gary Burghoff, Loretta Swit, McLean Stevenson, Alan Alda, Wayne Rogers, Mike Farrell and Larry Linville each appeared no fewer than three times. Alda, Farrell and Farr all played against Swit in regulation, and she and Stevenson were part of a special week in 1973. Mike and Loretta were the only co-stars to face each other on the Cullen 25. When Loretta faced Jamie for Classic TV week on Pyramid, it gave her the longest span of appearances on the show, 30+ years.
Hotel produced three regulars: Heidi Bohay, Michael Spound and Nathan Cook. In 1987, Nathan qualified two players for the $100,000 tournament in the same show, and the third player in that tournament qualified with Heidi.
Soap gave us Sal Viscuso, Billy Crystal, Jay Johnson and Robert Mandan as regulars, plus Jimmy Baio and Robert Urich. Robert Mandan holds the unusual distinction of having appeared as a representative of different shows where his TV offspring (Jimmy Baio, Mary Cadorette) also played Pyramid.
It seems to me that a number of Hee-Haw regulars also appeared frequently on Marshall Squares.
ETA: From The Mary Tyler Moore Show, Edward Asner was a Pyramid regular, Betty White did everything (but not Pyramid till '82) and Joyce Bulifante (Murray's wife) did a lot of shows too.