Ed O’Neill, for MARRIED WITH CHILDREN and MODERN FAMILY.
Bill Daily played second fiddle on a couple of successful sitcoms:
Roger Healey on “I Dream of Jeannie” and Howard Borden on “The Bob Newhart Show.”
And now for the b-level actors…
Suzanne Somers, with Three’s Company (first 5 of the 8 aired seasons) and Step by Step (7 seasons). In addition, IIRC from trade press at the time, she made a killing on the continuing syndication of the two seasons of She’s the Sheriff-- a made-for-syndication sitcom at a time when original content for syndication was very scanty.
It pushes the concept, but Bob Saget starred in Full House (8 seasons) and is narrator of How I Met Your Mother (7 seasons and counting), in addition to having hosted America’s Funniest Home Videos for 8 seasons.
I’m rather appalled that no one has mentioned Kelsey Grammer, from Cheers and Frasier. Not only successful, but two of the most successful of all time.
As mentioned above, Julia Louis-Dreyfus certainly qualifies with Old Christine.
David Spade was on Just Shoot Me for six years and Rules of Engagement for six and counting.
And then there’s the strange case of Valerie Harper: she racked up a bunch of Emmy nominations (and the occasional win) as the lead in RHODA before starring for a couple of seasons as the lead on VALERIE – which changed its name to VALERIE’S FAMILY and continued its six-season run without her. That’s just weird.
The OP said “no spinoffs” and specifically mentioned Cheers/Fraiser.
Jason Bateman Valerie & Arrested Development.
Bob Denver – The Many Loves of Dobie Gillis and Gilligan’s Island.
Cloris Leachman – The Mary Tyler Moore Show and Raising Hope
Gavin MacLeod – McHale’s Navy and The Mary Tyler Moore Show, with The Love Boat, if you consider that a sitcom.
Betty White – Mary Tyler Moore, The Golden Girls, and Hot in Cleveland
Richard Mulligan took home the Lead Actor Emmy for SOAP roughly a decade before doing likewise with EMPTY NEST.
Don Knotts created two rather iconic sitcom characters as Barney Fife (The Andy Griffith Show) and Mr. Furley (Three’s Company).
For that matter, there’s ole Opie Cunningham himself narrating Arrested Development. “No one was making fun of Andy Griffith. I can’t emphasize that enough.”
I was surprised that the OP discounted the entire cast of Seinfeld (New Adventures was the first thing I thought of) but allowed Becker.
What about Jason Bateman. He was in The Hogan Family and Arrested Development.
ETA, I see it was mentioned up thread (as Valerie).
Do animated shows count (e.g. Don Adams in “Get Smart” and “Inspector Gadget”)? Maybe not.
If you’re going to mention Al, mention Peggy and Kelly too.
Katey Sagal in 8 Simple Rules
Applegate in Samantha Who
How about Danny Devito in Taxi and Always Sunny in Philadelphia. Not the star of either but both are ensemble comedies that he was/is a major part of.
Courtney Cox, from FRIENDS to COUGAR TOWN.
He was also in The Good Guys, but that doesn’t meet the requirement of “successful”
Loved that show, but it was only on a season and a half.
Might Futurama be considered a sitcom? If so, Katey Sagal was in that.
The under-rated Jane Curtin: *SNL *was not a sitcom, but she had huge hits in Kate & Alli and 3rd Rock from the Sun.
Does Ricky Gervias qualify with The Office and Extras?