Just because they've been mentioned doesn't mean they shouldn't be mentioned again, but Seinfeld and Married w/Children.
While only 3 seasons aired in the 90's, I love the Golden Girls.
Also, while not technically a sitcom, Walker, Texas Ranger is hilarious!
Lot's of really great ones already mentioned that were staples of the 90's. For me the conversation starts and ends with Seinfeld. However, in the early 90's I really liked Saved By the Bell before I really got into Seinfeld.