To me, the 70s (ALL IN THE FAMILY, MARY HARTMAN, SANFORD & SON, WKRP, SOAP) and the 90s (ROSEANNE, SEINFELD, THE SIMPSONS, THE LARRY SANDERS SHOW) both make a strong case. Both eras saw some titanic shows take the whole form for a sharp turn and change it forever.
But for variety, depth, intelligence, and pure laughs, the high points of the 00s win it, for me. CURB YOUR ENTHUSIASM, THE OFFICE, ARRESTED DEVELOPMENT, 30 ROCK, MALCOLM IN THE MIDDLE, and the bulk of KING OF THE HILL and FUTURAMA - and that's just American TV. Throw in 00s UK shows like THE OFFICE, SAXONDALE, THE IN-BETWEENERS, EXTRAS, and SPACED and the choice is even more obvious.
The current decade is off to a good start, with MODERN FAMILY, EASTBOUND & DOWN, PARTY DOWN, BORED TO DEATH, and PARKS & RECREATION. But we'll see...
(By "sitcom", I mean any half-hour TV comedy with continuing characters - so not sketch comedy like MR. SHOW, THE CHAPPELLE SHOW, or THE KIDS IN THE HALL.)