I reject the thread title's implication that Friends and Seinfeld would obviously go on someone's list of favorite 90s sitcoms.
I only liked early Seinfeld. Once the "show about nothing" - with amusing little episodes about waiting for a table in a Chinese restaurant or trying to find your car in a mall parking lot - became a show about very over-the-top somethings and the 4 main characters grew increasingly annoying, I lost interest. (Annoying but funny is fine; annoying and unfunny leaves me cold.)
And I never thought Friends was any good but endured it for a couple of years because my girlfriend at the time enjoyed it. (I did love the Murder She Wrote episode that killed off an actor on the set of a very Friends-ish sitcom, though, so I guess Friends had some slight value.)
As for my actual favorite 90s sitcoms...
Murphy Brown - It had a few shaky episodes in the later years, but overall it was a solid comedy. Loved it when it was originally on and, watching it again now on Encore for the first time in years, it still holds up fairly well.
Roc - Sometimes serious; sometimes over-the-top silly. I love that after doing 1 episode from the 1st season live, they challenged themselves to do the whole next season live and actually pulled it off. Overall I thought it was an enjoyable little sitcom. (And there aren't that many Fox comedies I can say that about.)
I enjoyed the early years of Mad About You. It should have just ended once the couple reconciled and found out Jamie was pregnant. Most of what came after that was...unfortunate, to say the least.
I also really liked the 90s Britcoms As Time Goes By, Father Ted, and Waiting for God.
(I think of Designing Women as more of an 80s show and included it on the 80s favorites thread.)
Edited to add that, although I realize this isn't a sitcom, I wanted to mention that I think Mystery Science Theater 3000 is the funniest thing to come out of 90s TV and the show still holds up when watching the DVDs now.
Edited by Dave Lawrence, June 14 2014 - 03:57 PM.