Oh, Colin's taken my line already!
But 'best' for what? What was, for the time, controversial, socially-penetrating drama like 'Kathy Come Home', sharp satire like 'That Was the Week That Was', great writing, great production values, great acting? The original Forsythe Saga was fabulously well-acted, so was the 1970s series on Henry VIII. Blackadder is hysterically funny (to me) but possibly not universally. Then, 'best' escapism - surely 'Dallas' has to be given a straight 10 for that? I'm not sure that a decade isn't too long - television's a fast-moving medium, and seems to be evolving all the time, especially in terms of camerawork, CG effects, etc.
We seem to be in a golden phase of wildlife and 'environment' programmes, with all kinds of shots from amazing, innovative sites - overhead in the rain forest canopies, underwater, in deep caves, up the top of frozen mountains, etc., which are due to technical advances in equipment, as well as innovative thinking from the production teams. 'The Blue Planet' was filmed a few years ago, but it has some of the best production values, and along with National Geographic films, hard to beat for feeling 'right there'. Historical documentaries are nowadays incredibly interesting and bring in all of the latest discoveries (through forensics, etc.) to make the eras they depict positively glow with life. If only skool had been so interesting!