The fact that there are a handful of shows we're advocating, many of which have been off the air for a long time, speaks to their high calibre. That's why we haven't seen anyone mention "Charles in Charge" or "Facts of Life" (or a thousand other titles). So many of these great shows were pioneers of their time (e.g. Lucy, Honeymooners, All in the Family) you can't just pick one. We're certainly well on our way to a Top 50 list, though.
Thanks to those who have mentioned the Britcoms (Fawlty Towers, Yes Minister, Office, etc.). Several Britcoms have been the inspiration for American versions, so a tip of the derby to them as well.