For my part, i say, Buffy the vampire slayer...
why? 7 seasons (and the 8th on comics) of evolving and deeply researched characters, fun dialogs, and still forums and stuffs still active (even in french)...
Angel is not so good.
And i think friends is a good show... Kinda happy days of our times. Exept we live in economical crisis and they didn't...
For a sitcom I would say it has to have more than a dozen or so episodes. So a lot of english stuff I think is great (fawlty towers, pheonix nights, extras, young ones) would be counted out.
So more recently I would say Arrested development and seinfeld. But one of my all time favs and way ahead of it's time is Sgt Bilko, from the 50's starring Phil Silvers.
Those English shows you mentioned would be considered sitcoms. Just because they don't crank out 24 a year doesn't make it any less a sitcom than Two and a Half Men.
Another sitcom I liked was Newsradio. A funny cast, good writing, and the dynamic between Dave and Bill made it very entertaining. It's too bad it was moved 9 times, which is why it didn't have much of an audience.
I'm a casual fan of Two and a Half Men, especially the ones with hotties like Megan Fox, Jeri Ryan, or Emmanuelle Vaugier. And I always thought Charlie Sheen was pretty fun since the Hot Shot movies came out.
I used to love Frasier. The character of Niles especially. But it should have ended when Niles and Daphne got together. The episodes after that were boring.
But until then I thought the writing was on a par with The Simpsons.
I used to love Fresh Prince of Bel-Air when I was of a lesser age, but it looks pretty corny looking back on it. Still, Carlton doing the "white man dance" is still hilarious, and every time I hear "It's not unusual" I think of Carlton and Tom Jones singing together.