Bob Newhart had two huge starring roles and one more that lasted only a season.
Robert Culp had I Spy, Greatest American Hero and then a small regular role on Everybody Loves Raymond I believe.
Harry Morgan had many go back to the beginning of time.
“December Bride” … Pete Porter (111 episodes, 1954-1959)
“Pete and Gladys” … Pete Porter (72 episodes, 1960-1962)
“The Richard Boone Show” … Senator Clements / … (15 episodes, 1963-1964)
“Dragnet 1967” … Officer Bill Gannon / … (98 episodes, 1967-1970)
“MAS*H” … Col. Sherman T. Potter / … (180 episodes, 1974-1983) &
“After MASH” … Sherman T. Potter / … (20 episodes, 1983-1984)
Lucy of course had many. 4½ starring roles. The last one only lasted a half season.
James Gardner & Lee Majors are worth looking at. I think they had 3+.
I don’t know what counts as a hit, but Bill Cosby has “I Spy” (1965-1968), “The Bill Cosby Show (1969-1971)”, “The Cosby Show” (1984-1992), and “Cosby” (1996-2000), as well as “The Electric Company”, but I don’t know if you would call him the lead in “The Electric Company”. There was also “Fat Albert”. He was also in several other series, but they definitely weren’t hits (The Cosby Mysteries, You Bet Your Life, etc.)
Gotta give some points to Ron Howard. After roles in two (that I know of) major hit series, he went on to become an incredibly successful director. Not quite what the OP was looking for, but in the realm.
Richard Chamberlain, hands down. The guy had at least two hit TV series, including “Dr. Kildare”, and starred in at least 20 TV movies, including the megahit “Shogun”. He even did a TV movie of “The Bourne Identity” back in the 80s.
Actually, Newhart was also the star of another Bob Newhart Show in the 1960s, and one of the co-hosts of a short-lived variety show, The Entertainers. Not exactly hits, but that makes five star billings.
And then there was Bill Bixby.
My Favorite Martian (second-billed, behind Ray Walston) The Courtship of Eddie’s Father The Magician The Incredible Hulk Goodnight Beantown
PlusTrue Confessions, a daytime show, and shows on both PBS and Nickelodeon.
Lucille Ball was the lead in four separate series playing four different characters, three of which were hits:
I Love Lucy (Lucy Ricardo)
The Lucy Show (Lucille Carmichael)
Here’s Lucy (Lucy Carter)
Life With Lucy (Lucy Baker)
[Not sure what What Exit means by 4 1/2 hits. She also played Lucy Ricardo in The Luci-Desi Comedy Hour and Westinghouse Desilu Playhouse]
Bob Denver played one of the leads in five series, although only three lasted longer than a season:
The Many Loves of Dobie Gillis
The Good Guys
Far Out Space Nuts
Dick Van Dyke was the lead in two hits plus a third that didn’t make it plus a hit dramatic series:
The Dick Van Dyke Show
The New Dick Van Dyke Show
The Van Dyke Show
Fred Gwynne was a lead in:
Car 54, Where Are You
Television has a long, long history. Few people remember the early days any more.
I was counting counting the Luci-Desi Comedy Hour as a separate show and the Life with Lucy (13 episodes) as the ½.
However, Key Lime Guy quote implies that Lucy always basically played Lucy and Bob Newhart always played Bob. Danson had completely different characters at least. I would be surprised if he was really the only one though. It gets tricky with the words ‘successful’& ‘lead’ of course. I think John Ritter and Harry Anderson both would count at very least.
It’s pretty hard to argue that Lucille Ball played the same character in all her shows. She made a conscious effort to distinguish herself from “Lucy Ricardo.”
Same with Bob Newhart. Dr. Bob Hartley and Dick Loudon are kinda sorta the same but that’s more a limitation of Newhart’s acting.
This is one of those “facts” that depend on carefully hedging the definition down to where only one person qualifies.
Only person to star in two shows. Only person to star as different characters in two shows. Only person to star as different characters in two hit shows. Only person to star as different characters in two hit shows that lasted more than three seasons. Only person to star as different characters with different personalities in two hit shows that lasted more than three seasons. Only person over six feet tall to star as different characters with different personalities in two hit shows that lasted more than three seasons.
Do that long enough and you’re sure to have a winner. By any reasonable standards, however, Ted Danson has lots of company.