There's more than Clarke/Heinlein/Bradbury/Asimov out there. I just started my son on reading the Tripod Trilogy and I'm a big fan of John Christopher's YA books such as The Lotus Caves and The Guardians. Other books that had a big influence were Ben Bova's Exiles's Trilogy and The Dueling Machine; Harry Harrison's Make Room, Make Room, Stainless Steel Rat and Deathworld series, Brian Aldiss's Non-Stop (or Starship, which totally gives it away), Robert Silverberg's The City Inside, and Michael Moorcock's Breakfast In The Ruins and Behold The Man (this might p'o some parents given the subject matter) plus his Jerry Cornelius or Elric books if you're looking for some exciting, challenging stuff for teens.
