American Library Association: The Top Ten List of Frequently Challenged Books in 2012:

from The State of America's Libraries 2013 (American Libraries)

1. Captain Underpants (series), by Dav Pilkey 
2. The Absolutely True Diary of a Part-Time Indian, by Sherman Alexie 
3. Thirteen Reasons Why, by Jay Asher 
4. Fifty Shades of Grey, by E. L. James 
5. And Tango Makes Three, by Justin Richardson and Peter Parnell 
6. The Kite Runner, by Khaled Hosseini 
7. Looking for Alaska, by John Green 
8. Scary Stories (series), by Alvin Schwartz 
9. The Glass Castle, by Jeannette Walls 
10. Beloved, by Toni Morrison 

Only repeat from 2011 is Sherman Alexie's The Absolutely True Diary of a Part-Time Indian, which is highly rated by Amazon's customers.