Yes, kind of an open question, but I'm wanting opinions of shows you feel upped their game as time moved on OR ones that you can't believe continued to get renewed. I'm in the minority I'm sure, but I enjoyed the later seasons of Beaver and the Brady Bunch, as the kids were able to get into more 'adult' situations. Andy Griffith season one was borderline at best, until they got comfortable with where they were headed. I loved the pilot for Gomer, but the black and white season wasn't the best. It peaked midway through it's run. Jeannie was also better after season 1 and I wish they would have continued one addt'l season to play off the married couple angle. Hillbillies was better at the start, then they seemed to run out of ideas. To me Married w/ Children was an oddity. Not a fan of season 1, they hit their stride for a few seasons, then it turned into a cartoon version of itself.

Newhart started improving with the 2nd season.

I wish Newhart didn't switch over to film. I adore the videotape look and feel of season one.

Benson season 2 with Clayton Endicott III.

Facts of Life season 2 with Jo

Perfect Strangers season 3 with the Chicago Chronicle

Night Court season 3 with Markie Post as a regular

The Mary Tyler Moore Show was great from start to finish however in my opinion it got even better after the first season.

Later seasons of Family Ties, The Facts of Life, and Diff'rent Strokes are not as good as the earlier seasons. And of course Roseanne's final original season was a trainwreck of epic proportions.

Gilligan’s Island got improve treatment for Seasons 2 & 3

Happy Days got improved on majority basis from Seasons 2 to 8

The Partridge Family got a well deserved improvement beginning with the 2nd Season

MASH was great from seasons 1-3 and borderline from seasons 4-7 then they made a HUUUGE mistake and let Alan Alda take over the typewriter and directors chair; and every other week it became "...oh we're doctors and we're nurses and we save lives which makes us so good...and death is such a bummer...and war is hell..." and on and on...and Margrets reaction to every tragedy was "On god"

MASH was great from seasons 1-3 and borderline from seasons 4-7 then they made a HUUUGE mistake and let Alan Alda take over the typewriter and directors chair; and every other week it became "...oh we're doctors and we're nurses and we save lives which makes us so good...and death is such a bummer...and war is hell..." and on and on...and Margrets reaction to every tragedy was "On god"

YEP.

MASH was great from seasons 1-3 and borderline from seasons 4-7 then they made a HUUUGE mistake and let Alan Alda take over the typewriter and directors chair; and every other week it became "...oh we're doctors and we're nurses and we save lives which makes us so good...and death is such a bummer...and war is hell..." and on and on...and Margrets reaction to every tragedy was "On god"

Agreed.
A TV critic once said MASH began as a straight comedy interrupted by moments of unexpected drama, and ended as a straight drama interrupted by moments of unexpected comedy.
I would argue MASH was in fact THREE different shows.
Seasons 1-3. A combination of service comedy with the irreverence of the original film.
Seasons 4-7. An uneven mixture of comedy and drama, with the drama often overshadowing the comedy.
Seasons 8-11. A straight drama "lightened" by abysmal attempts at humor.