1 - FRIENDS
2 - Doctor Who (but not so much the recent series)
3 - Blackadder
4 - Fawtly towers
I'm not sure about the 5th, possibly big bang theory, but I can't really watch the same episode twice, unlike all the other ones I put down. So maybe the Inbetweeners... But it was not quite good enough... So I'm just going to say:
House (during seasons 2 to 4, it was the best show i've seen on tv)
Game of Thrones (still early days, but if the first season is anything to go by, it'll definitely be up there)
Honorable mentions would be Dexter, Arrested Development and Family Guy.
Depends on whether you're asking for the most popular/long running series or everyone's personal favourites. If it's the former then Dad's Army, Blackadder, Only Fools & Horses etc would probably be there, just for sheer widespread popularity to this day.
But going on personal favourites that I've actually watched at the time of airing:
Men Behaving Badly
Any quality murder mystery series e.g. Lewis, Midsomer Murders
Albeit all of the above have their drab episodes occasionally.