I think my favorite series would have to be Tomorrow series by John Marsden and Vampire Knight series by Matsuri Hino.
Even though I've only read the first book in the Tomorrow series, I totally loved it. I even really liked the movie version of it too, and not only because they were all Australians, lol. :P I never had the time to go out and buy the rest of the series, but I really loved Tomorrow, When the War Began and recommend it to everyone.
The Vampire Knight series immediately hooked me from the beginning with the mystery of Yuki's situation and awesome art. The art only became better as the series progressed, and the tension and drama and action also became better and better. Not to mention, the characters really played a big part in keeping you coming back for more in every volume.
Both are awesome series. I definitely recommend them.
(I just realized that my favorite series are from out of the US, lol.)