Are you asking which was our favourite era in general or which was our favourite episode? Because episode-wise, the way they executed the sitcom aspects, that would be the 60s for me. That was the one I enjoyed most as a sitcom episode. It felt like an episode of Bewitched or I Dream of Jeannie.
But my favourite sitcom era in general would be either the 80s or 90s as there were some classic sitcoms in that era. I don't know if they pulled off the 80s quite as well though, and by then there were some serious and more dramatic bits creeping in. And they more or less skipped over the 90s.