https://www.cbr.com/sitcoms-no-weak-seasons/

While some sitcoms are hit and miss, these ones are nearly perfect with no weak seasons in their entire run.

The Dick Van Dyke Show was consistently great.
I have only seen season 1 episodes of Gomer Pyle U.S.M.C., but am guessing all the seasons there are good.
Many disagree on this, but every season of M*A*S*H was good.
The Middle had bad episodes especially the Kirstie Alley one, but I still like all the seasons overall.
I Dream of Jeannie- Season 1 was slightly weaker but still worth watching.
Live action Corner Gas continued to have consistently great quality, miles better than any US sitcom from the same period.
Fawlty Towers, but then 2 seasons is a short run. same thing for The Munsters
The Patty Duke Show was consistently good in its 3 season run.
Every season of Coach was good.
I Love Lucy is close, like Jeannie season 1 was slightly weaker.