Of all time? Really hard to pick one. It depends what I'm looking for in a show. I'll give you a few of my fave picks though:
The Big Bang Theory, Bottom, Scrubs and Arrested Development for comedy covering most tastes (Geeky, slapstick, mainstream and um, different, in that order)
Doctor Who / Firefly for my sci fi fix
Walking Dead, horror, obviously
House and Veronica Mars for my drama (both of them still make me laugh/smile too)
Hopefully that's enough to go on for now!
Edit: Oh, and Russell Howard's Good News and Rhod Gilbert's work experience have both been excellent.