In these forums I've seen a few threads about TV shows that either died before their time or have long since lost their magic, but I don't think I've seen any for shows that got it spot on. That kept us all entertained for long enough but didn't outstay their welcome, and also managed to sign off without some cop out ending that everyone hated. So if you think you have one, name it. 
My choice would be Blackadder. 4 seasons of some of the best quality comedy ever seen on TV. It started off funny and only ever got funnier; and even after all the jokes the end scene was one of the most moving I have ever seen. I don't say this about many things, but it was, in my opinion, perfect. (There's also the fact that Blackadder was one of the two main shows that launched Hugh Laurie's career, so without Blackadder there may never have been a House, which is probably the best thing still on TV).
My choice would be Blackadder. 4 seasons of some of the best quality comedy ever seen on TV. It started off funny and only ever got funnier; and even after all the jokes the end scene was one of the most moving I have ever seen. I don't say this about many things, but it was, in my opinion, perfect. (There's also the fact that Blackadder was one of the two main shows that launched Hugh Laurie's career, so without Blackadder there may never have been a House, which is probably the best thing still on TV).