So we’re clear: I’m only looking for television series that (a) you consider great, (b) premiered while you were of an age to watch them, and (c) you never watched an entire episode of until after they were cancelled. In other words, if you adore Star Trek but only know it from reruns because you were born in 1980, find another thread. Contrarirwise, if you skipped Buffy the Vampire Slayer when it was on air because you thought the title was silly, then happened to catch an episode on DVD and became addicted to it, come in, sit a spell, and tell us all about it.
My own nomination: David Mamet’s The Unit. I can’t explain why I never saw this show during its original run. I love Mamet, I love Haysbert, and I love love love Regina Taylor; it should have been on my viewing schedule from the first episode. But somehow I missed it. It’s my own fault of course but I am going to blame the Welsh anyway.
Kind of similar to this current thread, but you have a few more distinctions. Babylon 5 - Never saw one episode and it’s one of the greatest shows of all time.
Firefly. I’d meant to watch it when it was on the air, and managed to finally catch an episode (which happened to be the very last episode aired). As there was already a lot of backstory going on with the characters, I had a hard time following it. When the DVDs came out, I was able to watch it from the start (the intended order, not the air-date order, of course), which worked much better.
Firefly - I forget why I missed it when it was first run, although I suspect it was some combination of it being on opposite something I was already watching or airing at an inconvenient time. I bought the DVD set solely on te basis of what I’d heard about it after it was off the air.
Buffy - While I had enjoyed the original movie, the premise didn’t seem like it would make a watchable series. Sometime later I’d heard from people whose opinion I respected that it was actually pretty good, but that it was hard to get into partway through. Then last November the Chiller channel started running it from the beginning, and I jumped at the chance to see it and was immediately hooked. I now have the complete series set sitting on the shelf.
Pretty predictable results IMO, with Buffy, Firefly, etc.
My pick is the Gilmore Girls. No, really. I’d come into the room and my wife and daughter were watching it, and I’d look at the screen, and think, “Holy shit, no way I’m watching this chick show.” Then I noticed Edward Herrmann was in it, and I sat down. (I’ve been a fan of Edward Herrmann since I first saw him in the Paper Chase movie in the early 70s.)
It’s smart as hell, very funny, and I’m going to marry Lauren Graham when I grow up. (I’m now halfway through season 5, after watching every episode from the pilot forward on DVD.)