Cheers was to some extent two shows: with Diane and without Diane. In general, the "with Diane" shows were far superior because of both the character tension between Sam and Diane ("two people who not only should not marry, [they] should never see one another again") and the fact the Shelly Long is a far more accomplished and stable actress than Kirstie Alley whose Rebecca Howe was a poorly defined character in part I suspect because Alley was so unstable (something that became quite apparent after the show went off the air).
That said, the without-Diane shows were greatly enhanced by the expanded presence of not only Frazier Crane but also (and especially) that of Lilith Sternin Crane, a tour de force for Bebe Neuwerth.
The writing on the show was consistently outstanding and probably its greatest strength. The acting, with the exception of the very uneven Kirstie Alley, was uniformly solid and Rhea Perlman was particularly strong in making the audience largely look past the fact that her character was almost entirely two-dimensional with little function other than delivering sarcastic punch lines. The directing too was outstanding. Because the other characters (excepting the aforementioned Rebecca Howe) were so well drawn and well acted, the viewer (at least this viewer) felt a sense of watching real people cope with life. The show's lessons (or morals) were subtle and subtly delivered with wonderful positive humor ("put your faith in God, I know I'm going to" spoken by the head nun of the convent as advice to Diane and a comment on eating Diane's "creative" entree). The show was comforting and life affirming, imperfect people accepting other imperfect people. Whatever triumphs or tragedy befell them, there would always be the place "where everybody knows your name."
Perhaps most touching was the show's genuine fondness for and remembrance of Nicholas Colasanto (Coach). In (at least) two episodes, he is memorialized by name and specific reference (the first show of the season after his death and the memorable Thanksgiving episode) and, in perhaps the most touching coda of any TV series, Sam's last act in the last episode is to straighten the picture of Geronimo that came from Colasanto's house and became a permanent part of the set after his death.
It is in my all-time top five with Seinfeld, Scrubs, I Love Lucy, and (especially the early years') Cosby.