My list:
The Simpsons - I know everyone complains that it's not as good as it used to be, but it's still brilliant and really it sweeps all other sitcoms before it. It's one of those programmes that, on paper, shouldn't have really worked, but somehow did. It's kind of the TV equivalent of Peanuts in that it's not just a set of comic characters but a whole comic world that is endlessly adaptable and versatile.
Hancock's half-hour - not quite the first sitcom but the first great one. It was doing post-modern self-awareness "nothing happening chic" while Jerry Seinfeld was a toddler.
Red Dwarf - was consistently brilliant until one of the writers left and it went sharply downhill. The best thing about it was that it looked like a sci-fi show but was really character-driven - the best episodes were ones in which nothing really happened.
One foot in the grave - I think this was really the first mainstream sitcom to mix in sharp dashes of very black humour and genuine cruelty into what looked like it was going to be a gentle suburban comedy. The writing was just so clever. I thought at the time the author should write a detective series, and then he did.
Spaced - I didn't see this at the time, but only later, but it's still kind of era-defining. Generation-defining, perhaps, no matter when you actually saw it.
Fraser - the most effortlessly cool show of all time. This programme shouldn't have worked, for so many reasons, but it did.
Malcolm in the middle - I always loved this, just brilliantly done. Hal is one of the great "comedy dads" of all time.
30 Rock - only recently discovered this but I really, really like it. It just has a friendly sort of feel to it.
Outnumbered - only just discovered this too but I think it will be an enduring classic. This programme is basically a long ad for contraception. The expression on Hugh Dennis' face all the time is priceless.
I have only seen a few early episodes of Seinfeld, a couple of years ago, and I thought it looked brilliant and not dated at all, but I'd have to see more to judge. That's a programme I'd really like a complete box set of.