To echo several others here, Harry Potter, of course, is one of my favorite series. I also love all the series by David Eddings...I guess if i had to pick one, I would say The Mallorean
is my favorite. Or maybe The Belgariad
Twilight is all right; I don't think it's fantastic, but I enjoy that series. Oh, I also the Wheel of Time series, by Robert Jordan. New book is coming out next month!