Harry Turtledove is a good author, he writes adult fiction and fantasy (Adult meaning that there are tasteful sexual scenes). The darkness series is probably one of my favorite series.
However if I had to restrict myself to young adult series.
Pendragon was really popular when I was in high school and are pretty damn good, finished everything but the last book.
The first few Maximum Ride books are interesting but horribly cliched and predictable for anyone who reads science fiction/fantasy regularly. The concept and characters are good on their own but together they are meh. My sister loves them but I don't, they aren't bad but not great.
Also a manga series for some reason. I checked it out at a book store, art was good and for some reason everything I hated about the book lends itself great to the visual format.
Vampirates is good too. It is exactly what it sounds like, Vampire Pirates. Not as bad as one would think. Again major plot points are predictable if you do much fantasy reading but makes up for it.
If you can find it I highly recommend finding a book called The Transall Saga though it goes by the name Blue Light if you are in the UK. Standalone book, 240 pages, written by Gary Paulsen (Hatchet, Soldiers Heart, many other novels) in 1998. Damn fine book and one of the few books I plan on keeping with me for a long time. Might be hard to come by since it never was all that popular.