Ok, has anyone heard of the Pendragon series? It's AWESOME! I am not kidding, after I read the first one I was like, "ohmigod is there any more?" As it turns out, there are going to be 10 total, but only 5 have come out so far (in order: The Merchant of Death, The Lost City of Faar, The Never War, The Reality Bug, and Black Water). Not that I'm complaining.
They're probably classified as fantasy, with a little sci-fi like time traveling, although it's not really time traveling, it's traveling to different "worlds" (territories). It follows the course of Bobby Pendragon's adventures. He's a 15 year old who just found out he's a "traveler" (he can flume to different territories). The catch is, as he visits each territory, it's on the verge of a great turning point, which could either help the territory or set all hell lose. Bobby's job is to ensure the territory remains protected during the turning point, but his nemesis, St. Dane, is there to ensure the opposite. Even if the plot sounds simple, I assure you, it's not.
D.J. MacHale totally puts his mind into that of his fictional 15-year-old hero, so teens' intelligence is the last thing insulted in this series. It's got humor, adventure, mystery, and a little romance.
What more could you want?