Not a huge fan of sitcoms in general... but:
BEST:
Frasier: One of my favourite shows ever. It almost feels a bit like theatre in parts, with the overdramatic acting and the chemistry between the main characters (especially Kelsey Grammer and David Hyde Pierce).
While i agree that the later seasons lost a bit quality i still think that they are way above most other shows i have seen. The last season even has a few of my favourite episodes.
Community: Doesn't technically count i guess, but its my other favourite comedic show. Any show that has Chevy Chase dressed as the Beastmaster on drugs does something right :D
Its also a great example of nerdy humour done right i think. While in TBBT you just laugh -at- the characters and their antics here its much more embraced and also often a clever way to hide more serious matters, while still being funny. Great cast too.
WORST:
Big Bang Theory: pathetic, unlikable characters (i think Amy is the only decent person out of all of them), lame jokes (no, a reference for the sake of having a reference isn't funny), very repetitive humour too, obnoxious laugh track, overall awful.
Two and a half man: Just.. no.
Full House: So. Lame.
timppu: It is similar as with Frasier, where the point was for common people to laugh at the manners etc. of cultural "upper class" people. Like how their hobbies include wine-tasting, going to manicure etc...
Agree with TBBT, Frasier not so much. On Cheers the Frasier character was like that but on the actual show its rarely the "upper class" things they do that are the main source for humour but more their desperate attempts to belong to what they consider upper class. The episode with the manicure is a nice example, they could be perfectly happy with what they have got but they crave so much to be even more elitist that they end up on the street next to a dumpster. Its more about their vanity than anything else, in my opinion