I keep hearing good things about the Inheritance trilogy by Christopher Paolini; the Twilight series by Stephenie Meyer and others. I haven't read them myself, so I'm only basing this on reviews.
Some classics you may enjoy include:
Jane Eyre by Charlotte Bronte;
Wuthering Heights by Emily Bronte;
Dracula by Bram Stoker;
Pride and Prejudice or Northanger Abbey by Jane Austen (or any of her other 4 novels);
Great Expectations by Charles Dickens (or any of his others);
The Hobbit by J.R.R. Tolkien (or his Lord of the Rings series);
The Master of Ballantrae; Kidnapped; Treasure Island (perhaps even Dr. Jekyll and Mr. Hyde...also known as The Strange Case of Dr. Jekyll and Mr. Hyde) by Robert Louis Stevenson;
1984 or Animal Farm by George Orwell;
The War of the Worlds by H.G. Wells;
Little Women by Louisa May Alcott
and
Anne of Green Gables by L.M. Montgomery.
For more modern teen fiction, some interesting lists can be found at:
http://www.teenreads.com/
http://www.wtmlib.com/ya/ya_myst.htm
http://www.library.org.nz/teens-zone/teennovels.html
http://www.teenink.com/Fiction/
http://www.pburglib.org/humorous.htm
http://www.pburglib.org/horror-suspense.htm
http://www.bookspot.com/youngadultfeature.htm
http://www.seemore.mi.org/booklists/fiction.html
http://www.mysterynet.com/edgars/previous/youngadult.shtml
I hope you find something you enjoy!