Note that there's still lots of popular shows I haven't seen (original Twilight Zone, MI5, new BattleStar Galactica, The Wire) and this decade's young, so this list may change over time.
1950s: Adventures of Superman. This one wouldn't make my list of all time favourite 100 shows, but the pickings are a bit slim this decade and this was a solid enough all ages adventure show.
1960s: The Prisoner: A former secreet agent gets kiddnapped and placed in a mysterious Village where people try to find out why he resigned while he tries to escape. An allegory about personal freedom.
1970s: Fawly Towers: Sure there may be a more sophisticated shows this decade: but this series about an arrogant yet insecure hotel manager (loosely based on a real one) is a lot of fun and timeless.
1980s: The Singing Detective: A man suffering from psoriasis starts having musical hallucinations that blend in with reality. 6-episode mini-series.
1990s: South Park: This animated series abouit small town children is still airing and arguably its most recent stuff is even better than this era; I placed it here anyway because of the quality of other shows in later decades; this is the only decade where for me it beats out other shows then airing. Sure it's rude and crude but it's also very satirical with great satire on small towns, celbrities and whatever other targets attract the writers' attention.
2000s: The Shield: This series about a corrupt cop asks difficult questions about morality. In a rough neighbourhood is it okay to enlist the aid of someone who skirts the law to protect the area from even worse elements?
2010s (so far): The Walking Dead: A group of people thrown togethjer by necessity try to maintain their humanity while surviving in a zombie-ravaged world.