Not counting rereads, which actually formed the bulk of my 2012 reading, my picks are Frank: The Voice by James Kaplan for nonfiction, and Dare Me by Megan Abbott for fiction.
Frank is a biography of Frank Sinatra, covering the ups and downs of his life and career until his Oscar win for From Here to Eternity. Kaplan presents a well-rounded portrait of a complex person, and I'm looking forward to the second volume.
Dare Me is a noir tale set in the world of high school cheerleaders. The story is an irresistible mix of competition and friendship and jealousy and murder. Abbott is a fascinating and artful writer.