I nominate
Huckleberry Finn by
Mark Twain.
It's been quite a while since I have read of its being challenged, but back in the '70s it was an annual event for parents of public school children to object to its presence in the schools' libraries because it includes the use of the "n-word."
I read it when I was in the eighth grade, and I'm sure that as an adult I would get much more out of it now.
It's free at MobileRead's Patricia Clark Library.
Kindle
https://www.mobileread.com/forums/showthread.php?t=40303
or
https://www.mobileread.com/forums/showthread.php?t=14089
ePub
https://www.mobileread.com/forums/sho...d.php?t=193287