Lots of excellent suggestions - Heinlein, Tolkien, Vonnegut, Card, Herbert, Pratchett, Dumas - I loved all of these except Pratchett and Card and only because I was an adult before finding them.
Jules Verne. H.G. Wells. Gene Wolfe's Sevarian series.
Keith Laumer's Retief books.
Poul Anderson. Vernor Vinge. Guy Gavriel Kay writes beautifully.
Known Space series by Larry Niven. Read "Protector", the Ringworld books, or the collection "Known Space".
Robert Jordans Wheel of Time series.
Run, don't walk to Miles Vorkosigan books by Lois McMaster Bujold.
David Drake writes great fantasy adventure and space opera books.
Jack Vance's "Cugel's Saga" is great fun.
Walter M. Millers Liebowitz books.
Joe Haldemen.
C.J. Cherryh can't be recommeded too highly.
Mervyn Peake's Gormengast books.
The War Against the Chtorr is a series of novels being written by David Gerrold.
Don't be afraid of historical authors. Mary Stewart, Cecelia Holland, Mary Renault, Dorothy Dunnet.
T.H. White's "Once and Future King", or the Claudius books.
Bernard Cornwell wrote the Sharpe series.
George MacDonald Fraser's Flashman books should be in everyone's library. Awesome stuff, and totally engaging.
Mark Twain. Seriously. He wrote a lot more than Huck Finn.
I'd better stop now
