Last great sitcom?

The last great sitcoms all started in the 1970s, and were gone by the early 1980s:

1) M*A*S*H
2) Welcome Back, Kotter
3) Taxi
4) WKRP in Cincinnati

Starting with the 1980s, sitcoms became far too self-aware, self-regarding, and self-satisfied: just an over-all vibe of "Nobody's more cool than us!"

I Love Lucy
It’s been all downhill since then.

Seinfeld
Living Single
Friends

Frasier is the latest once I've seen and liked. Just never got into any newer ones. ¯\_(ツ)_/¯

I should add my two cents. Newest sitcom I loved, it got cancelled. The great indoors. It was just getting good near the end of the first season. Then, poof...gone.

Community (2009-2015, available on Prime) starring Joel McHale, who later starred in The Great Indoors. (You are correct, T.G.I. should have continued.)

Everybody Loves Raymond.

How I met your mother.

You can argue whether to call it great or not. It's basically Friends 2.0. However, you can safely say that as a minimum, it's extremely good. No sitcom has come close to that level since then. There were many great sitcoms before HIMYM, not a single one after it.