re: Is Cheers the best sitcom of all time?

You almost have to divide comedies by era - you have:

The golden age stuff, Lucy, Honeymooners, Ozzie and Harriet, etc.,

The 60s "rural" era, Andy Griffith, Green Acres, Gilligan, etc.,

The "transitional" era, (notably the duelling family comedies Brady Bunch and Partridge Family), almost a mini-era or spacer,

The "gritty" 70s, somewhat more urban, All in the Family, Sanford, Good Times, Jeffersons, M*A*S*H, Taxi, WKRP, SOAP, etc.,

The 80s, "cheesy" era, epitomized primarily by the main "must see" lineup of Cosby, Cheers, Night Court,

Then the long stretch where Seinfeld and Friends dominated before the modern rebirth of comedy, Modern Family, 2 1/2 Men, BBT, etc.

There are great comedies at every era. Hard to pick 1. Cheers is one of my absolute favorites.