Many people would probably throw their lot in with Card's Ender's Game or maybe something less spacy like Children of Men. Some will probably go old-school with Stranger in a Strange Land or A Childhood's End. A Brave New World? However, what many of those novels lack is a modern, topical tale written on the grandest scale imaginable. My choice, of course, is Dune, by Frank Herbert.
Herbert Had To Have Been Clairvoyant
Unlike that regrettable series, however, it deserves the line.