I recently finished the first season of Modern Family in like two days. It's soooooo good! I think it's a must for sitcom lovers. I am hoping for more.
Seinfeld is a must and then you can transition into Curb Your Enthusiasm...starring Larry David..the co-creator of Seinfeld.
also Arrested Development is my favorite sitcom of all time...its sad that it only lasted 3 seasons Always Sunny in Philadelphia is definitely up there on my list as well
Married ... with Children M.A.S.H Fresh Prince of Bel´air Spin City Black Books Yes, Minister (Yes, Prime Minister) Red Dwarf These are the first ones to come on my mind. If you dont like british humour, skip last three
When it comes to sitcoms nothing holds up to It's Always Sunny in Philadelphia and Arrested Development.
It's Always Sunny seems to be really hit or miss though. From personal experience most people either hail it as the funniest show ever or just get a headache from it. The same may be somewhat true for Arrested development too actually, for that one I guess its just too dense for some people to get into (as in layers upon layers of recurring jokes and references and confusing plots).
Not sure if it's already mentioned, but Coupling (the british version) is one of the better comedy shows ever made. Last season sucked, but the first three are pure gold.
News Radio is also exceptionally good altough pretty much not known by the general public. Really a shame.
I would also avoid Big Bang Theory at any cost. It's cliche-ridden, predictable and just overall lame as shit