I'm listing my favorite shows here, but there are a lot (especially from HBO) that I haven't gotten to yet. I've only recently discovered the wonders that can be the TV format, honestly. Though I'm studying it closely now that I have.
Also, I've been watching only Joss Whedon and the BBC shows for the past few months, and pretty much just sci-fi / fantasy, so this is a really skewed list. So I'm in no way suggesting this is a list of the best shows of all time. I don't think I could do that. Actually, even if I'd seen enough TV to start making such a list, I'm not sure it'd be fair. You'd probably have to go by genre / mode.
1. Doctor Who – as an entirety, it's probably the greatest show ever, though I readily admit it's been terrible many times in its history, and in fact is currently insufferably mediocre. Still, its infinitely flexible premise and its history give it so much potential. And, like the Doctor himself, it's immortal. It can always rebound from a terrible period and be awesome.
2. Angel – And now for the deluge of everything Whedon. I'll talk about Angel in comparison to Buffy below.
3. Buffy the Vampire Slayer – If any of you haven't seen this show, do yourself a favor and watch it next. Especially if you love Firefly. Buffy and Angel were the only chance Whedon ever got to show what he could do in the full run with a TV show – and
holy shit is the guy brilliant. He invented so much, and other things he's done better than anything else – the "Big Bad" episodic season-arc structure, for instance. For instance, he has an amazing ability to make episodes which introduce a stand-alone kind of conflict which pushes the show's characters exactly in the right direction the season arc needs them to go. Not only that, but his shows never got stale – they always evolved, developing their thematic concerns while simultaneously staying true to them.
As to why I ranked Angel above Buffy: it's a difficult choice. There are certain episodes of Buffy which are unlike anything else I've seen in any art form, much less just TV.
"The Body" – an episode in which Buffy discovers her mother's dead body – is the most obvious example. The only art work I've experienced which conveys one's initial reaction to the death of a loved one so vividly has been War and Peace and The Death of Ivan Ilyich – both by Leo fucking Tolstoy. Whedon manages to make you feel as if Buffy's mother were a real person, and you're reacting to her death right along with her. I'm not ashamed to admit that that episode made me cry – and nothing makes me cry. I just watched it again with a couple friends, one of which doesn't even watch Buffy, but who was still captivated, and later left my apartment completely devastated. It's hard to convey how good this episode is. I might go so far as to call it the best episode of TV ever.
And while that isn't as true with Angel, I still think it's more consistently good than Buffy, and in it dealt with season-arcs (and mini-arcs) better. And it's just much more thematically rich. But I'm biased, because I was a fan of Angel as a child, and it's kind of become a major part of my psyche.
4. Torchwood – Honestly, there are moments when Torchwood has been better than Doctor Who. It's a show whose brilliance grows on you. You don't realize just how great the characters are until you lose them and/or until you see it a second time. It's very Whedon-esque that way.
Of course, I'm not counting the atrocity that was Miracle Day, because that was just stupid.
5. Firefly – this might have been counted higher if it had had a longer run. It's tragic, really. It's the only time I've seen Whedon do sci-fi as well as he could.
I think I'll stop now.