Paul Reiser was in My Two Dads (3 seasons) and then Mad About You (7 seasons). His latest sitcom attempt was canceled after airing only two episodes.
Ellen Degeneres in Ellen and now her daytime show, which certainly features her comic abilities.
Jimmy Fallon on SNL and now Late Night.
Also Chevy Chase on SNL and now on that regrettable comedy…Community(?).
Tina Fey even more so, with 30 ROCK.
Sarah Chalke was Becky Conner in 70 episodes of Roseanne and Elliot Reid in 173 episodes of Scrubs. She also had a minor role in a few episodes of How I Met your Mother as well.
Some British ones :
Leonard Rossiter - Rising Damp, The Fall and Rise of Reginald Perrin
Richard Beckinsale (Kate’s dad, died at 31) - Rising Damp, Porridge
Ronnie Barker - Porridge, *Open All Hours *and one half of the very popular sketch show The Two Ronnies
David Jason - Open All Hours, A Sharp Intake of Breath, Only Fools and Horses, Darling Buds of May, and of course Danger Mouse. Jason likely had more successful sitcoms, these are the ones that spring to mind.
I would not say the entire Seinfeld cast failed at what you suggest. Julia Louis Dreyfuss’s sitcom The New Adventures of Old Christine lasted 3 or 4 years; that’s long enough to count as successful.
Bill Cosby headlined three successful sitcoms.
Melissa Joan Hart starred in both Clarissa Explains It All and Sabrina, the Teenage Witch. ETA: It might be too early to declare Melissa & Joey success, though. But Joey Lawrence was in both Blossom and Brotherly Love!
If animated sitcoms are counting, it hurts to say, but Seth MacFarlane should count for Family Guy and American Dad. Both shows are quite successful in syndication. I’m not really a fan though. However, I think including voice actors might not be what you’re looking for, OP.
Jackée Harry starred in 227 and had a prominent role as the mom Sister, Sister. Tia Mowry is having success with The Game, which has been around for 6 years now. Her little brother, Tahj, starred in Smart Guy and Kim Possible. Also from KP, Will Friedle, who was Eric Matthews in Boy Meets World. I guess I shouldn’t be surpised that MJH was in an episode of both BMW and Batman Beyond with Will Friedle.
I miss the 90s 
Sarah Chalke - Roseanne and Scrubs
She also got five consecutive Emmy nominations as Best Actress for that one, including her first-season win.
Going back a ways:
Gale Storm in “My Little Margie” (1952-55) and “The Gale Storm Show” (1956-60, aka “Oh Susanna”).
Ann Sothern in “Private Secretary” (1953-57) and “The Ann Sothern Show” (1958-61).
Rowan Atkinson was in “Blackadder” and “Mr. Bean.”
John Cleese went from MONTY PYTHON to FAWLTY TOWERS.
Katey Sagal is in that. Season seven starts in June on Comedy Central.
James Garner went from Maverick to Rockford Files, both fairly tongue-in-cheek. I could watch that guy drink a cup of coffee.
Jerry Van Dyke in My mother the car and Coach.
He was also in 8 Simple Rules following John Ritter’s death.
Will Arnett also has a recurring role on 30 Rock.
So was Al Lewis.
Judd Hirsch was in, IIRC, 4 sitcoms, one with Bob Newhart. Taxi and Dear John were both very successful.
“regrettable”?!? :mad:
I think you meant to say, “the funniest sitcom currently on TV, Community.”
Don’t know if he’s been mentioned yet, but Sherman Hemsley had 11 seasons on The Jeffersons (which is of course a spin-off of All in the Family) and 5 seasons on Amen.