I used to work as MD for a singer, Madeline Bell who hailed from Newark, New Jersey. She told me that the word she hated was 'Nigra'.
She also sent out copies of Little Black Sambo as a Christmas card to friends one year... Although as has been said above, the word "nigger" has been a hated racial epithet for all of the 20th Century, at the same time it has been used fairly innocently - N. Minstrels, the colour N. Brown and so on.
Finally, a white musician I knew told me that he was in a bar in the US, that was largely patronized by black people. Next to him at the bar was a man who was talking loudly to his friend, and calling him 'Hey, N. - blah blah blah...' he then turned and said, 'Oh, sorry man', apologizing to the white musician for his use of the word!