Right now it's a tie between Supernatural and Legend of the Seeker.
Supernatural I've been with for the last four seasons now, from the second season which is where I first started watching the show. Although some may know that I'm still kind of torn on where I stand about the show as of right now and my opinions about the direction it's taking. That may change when the new season starts, I don't know. And of course I immediately was hooked onto Legend of the Seeker which has been my recent newest obsession. I consider LOTS to be something that gives me what some shows haven't done lately, which is that feeling of satisfactory fulfillment with the story and the characters and a great kind of escapism. Of course, I've already gushed about the show before and probably will continue to do so in the future, but yeah. Those are my current television obsessions as of right now. Things are most likely to change though depending, since my obsessions and objectives change constantly. Especially with all the other shows I have on my queue, it's only a matter of time before another new obsession kicks in.
Runner Up: Caprica. While I do love the show so far it's taking far too long to finish the first season due to this unnecessary midseason hiatus. So I really can't say it's an obsession yet, but it's getting there.
Masterlist of days here.