Supernatural, back in s4. I just loved the tale of the two brothers.
And then Criminal Minds, because I'd read an SPN crossover with the series, and I wanted to see the characters that I liked in the crossover.
NCIS because I had to write a crossover for a ficathon, and I really liked the prompt involving NCIS. I ended up liking the characters enough to watch the entire series in less than a month.
Leverage, the Listener, Avatar, ... and many others because I heard good things about them...