A Game of Thrones - Season 2
If you don't know, you should. Get thee to a Netflix - there's still time to get caught up before tomorrow night. Based on George R.R. Martin's best selling (and seemingly never ending) series, A Song of Ice and Fire, this show is doing for fantasy in this decade what Lord of the Rings did for the 00s.
River Monsters - Season 4
If you had told me five years ago that one of my most anticipated shows would be a fishing show, I'd have told you you were nuts. But Jeremy Wade hosts an aggressively addictive show - an "extreme angler", he travels around the world, tracking down creatures that attack humans. Along the way he meets with local fishermen, and learns their ways of doing things. He's reeled in everything from Bullhead Sharks to Goliath Tigerfish, Giant Manta Rays to Candiru. It's on Animal Planet, and you'll be just as surprised as I was to find that it's more than worth your time.
Avengers: Earth's Mightiest Heroes - Season 2
If you've been waiting for Marvel's answer to the unparallelled awesomeness that was Timm/Dini's Justice League (Unlimited), this is it! It's got everything you could want from a comic book cartoon - guest stars galore, and action and set pieces on a scale that no live action movie can afford. Written by solid comics scribe Christopher Yost (who also wrote the late, much lamented Wolverine and the X-Men cartoon), the first season was solid - mixing the Ultimate and 616 continuity, taking the best of both.
The Killing - Season 2
It's been compared to The Wire, and from one perspective that's certainly true - both present rich, complex worlds, and take a multifaceted approaching to studying the effects of crime. But while the Wire was very intellectual, almost coldly so, and was focused on institutions, the Killing is a very personal, very emotional, very raw story. It takes the time to examine how the titular Killing affects the lives of people from the immediate family to politicians running for office. Based on a Danish TV series of the same name, the second season is essentially the second half of a 20-odd hour movie, so go back and find the first season first, or you'll be lost. This is also the American breakout performance for Danish-born Joel Kinnaman, who, as a result of his work on the Killing as the creepy/awesome Stephen Holder, was cast in Safe House and the Girl With the Dragon Tattoo, and will be playing Murphy/Robocop in the upcoming remake.
I'm going to put a trailer up for Season 1, to avoid spoilers. It's yet another big hit for AMC, to go with Mad Men, Walking Dead, and Breaking Bad. Do yourself a favor and check it out.