Here are some of the books I remember liking most from my high school years. You'll notice a high proportion of fantasy, science fiction, and historical fiction (or classics that took place well before my time).
-The Princess Bride by William Goldman (The movie is one of my favorites of all time. The book is quite different, but also very good.)
-The Adventures of Huckleberry Finn by Mark Twain
-The Lord of the Flies by William Golding (Boy did my friends and I have a lot of discussions on this one)
-And Then There Were None by Agatha Christie
-Dealing with Dragons by Patricia C. Wrede (And Caught in Crystal, a less comedic fantasy)
-House of Stairs by William Sleator
-The Hitchhiker's Guide to the Galaxy by Douglas Adams (And The Restaurant at the End of the Universe)
-Emily of New Moon by L.M. Montgomery
-Ender's Game by Orson Scott Card (And Ender's Shadow)
-Rebecca by Daphne DuMaurier
-Emma by Jane Austen (and all the others, really, with Pride and Prejudice a close second)
-The Scarlet Pimpernel by Baroness Orczy