The Killing (U.S.) has gotten a lot of flack from people who were pissed about the non-resolution of the plot last season. I love it. Linden and Holder are gonna solve this case, and whichever powerful a-hole who thinks he is safe from authority is going down. My biggest fear is that one of our beloved main characters won't survive the season.
Nurse Jackie is, in my opinion, one of the smartest-written shows out there, and such a dead-on examination of addiction. I love, love, love it. We watched the first three episodes of the new season On Demand the other day, so now will have to wait a few weeks for the next new episode (but will probably watch 2 and 3 over again as they release on Sundays). Jackie goes, sort of, to rehab, and is clean for the first time since she had her first child. In my opinion, it's not gonna stick, but still I root for Jackie.
Another show that makes the all-time list just finished its 2nd season - Shameless (U.S). If you're not watching this one, you really should. It's so sad/wonderful. I think the show is mostly about surviving your parents, but the characters are so awesome it could be about a dozen other things and I'd still be hooked.