I'm assuming you enjoyed Supernatural and Doctor Who? If so, you might also like:
The Twilight Zone - Each episode is a self-contained sci-fi short story. This is one of the first true sci-fi television series and is a true classic.
The X-Files - One of the first series to adopt the formula of intermixing Twilight Zone-esque monster-of-the-week episodes with story arc episodes. It's a bit spottier than some of my other recommendations will be (some episodes were really, really bad and the series-long arc ended up making very little sense, but it featured a ton of truly excellent episodes). Watch the first four or so episodes, and if you're enjoying it by that point, keep watching.
Firefly - If you haven't watched it yet, watch it. There are only fourteen episodes, so it's not like it's a huge time commitment. Plus, it brings me to my next recommendation.
Buffy the Vampire Slayer - Sort of similar to the X-Files or Supernatural in its mixture of monster-of-the-week and seasonal/series-long arcs, although it's a lot more humorous (IMHO) than either of those shows. Where Buffy really shines, though, is in how it takes advantage of the serialized nature of television programs to develop its characters over the course of seven seasons. Of course, seven seasons is a big time commitment. Watch Firefly first; if you enjoy that show's writing style, watch Buffy. Be warned that it takes some time to find its feet; the first season is very campy and doesn't really encapsulate what people love about Buffy. I'd say give it until the end of the second season; if you're not enjoying it by then, it's safe to say you won't like the show.
Torchwood - You said you've watched Doctor Who? Torchwood is a Doctor Who spinoff.
