It is absolutely criminal that Doctor Who isn't listed here yet. I wasn't as big a fan of the first few seasons of the revived show, but you can very easily start at the beginning of the last season (series 5) - there are relatively few references to previous seasons (virtually none) and it's probably best that you start with the current Doctor rather than starting with another one and ending up bitter and jaded when you come to the point where that actor leaves the show (this is a very common problem within the DW fandom).
The dialogue is brilliant, the actors are great (please don't take this as an invitation to whine about how you liked a previous Doctor more: see above), and the plotting is downright magical. The point at which I suggested you start watching is, beyond being the main introduction to the current Doctor, also the point at which Steven Moffat took over writing for the show. And that man can fucking write. He doesn't have quite as few pithy one-liners as Whedon, but I much prefer his plots and general dialogue (and the dialogue is noticeably easier on the actors too - Whedon actors tend to stumble pretty egregiously for a while getting used to his unique writing style).
It's also worth checking out another completely different series Moffat did recently called Sherlock, though I realise that's not scifi. It's modern-day Sherlock Holmes that is, contra what you would expect given the setting, a lot more faithful to the character in the books than most other adaptations I've seen. The writing is really engaging and the actors are again very well-chosen (Martin Freeman is Watson!).
In terms of Whedon, you might also like Dollhouse. I don't think Buffy has aged quite as well if you discount the nostalgia factor, so I wouldn't recommend that too strongly, though it still has its moments. Also, every person should be forced to watch Dr. Horrible, Clockwork Orange-style if necessary.
