I'm not a huge fan of television overall, as I find most shows far too demanding for too little reward. But, there are two that always stick out to me for their excellence: Mad Men and Batman: The Animated Series. I find myself going through both series at least once every few years. B:TAS is great because it actually understands its source material and makes the best of it while taking full advantage of its television medium. Mad Men is just breathtakingly good. I could go on and on if someone dared me.
As for my third one? That varies from day to day. Daredevil is excellent, even if its second season falters a bit and the beginning of the third season relies on the mediocre The Defenders show for important context. For sitcoms, All in the Family is a riot and one of the few I enjoy. For childhood nostalgia, The Real Ghostbusters is #1. Utterly adored that cartoon as a kid, and some of the best episodes still hold up to this day.