“I have never met a man so ignorant that I could not learn something from him.”

General, The Idiot (1868–9)

The Idiot (1868–9)
Context: I have never in my life met a man like him for noble simplicity, and boundless truthfulness. I understood from the way he talked that anyone who chose could deceive him, and that he would forgive anyone afterwards who had deceived him, and that was why I grew to love him.

Source: Dear John

Not found in Twain's works, this was attributed to him in Reader's Digest (September 1939): no prior attribution known. Mark Twain’s father died when Twain was eleven years old.
Disputed
Variant: When I was a boy of fourteen, my father was so ignorant I could hardly stand to have the old man around. But when I got to be twenty-one, I was astonished at how much the old man had learned in seven years.

Georgina Howell, The Demanding Nastassia Kinski http://news.google.com/newspapers?nid=1309&dat=19860102&id=MQROAAAAIBAJ&sjid=MJwDAAAAIBAJ&pg=6682,494045, New Straits Times, January 2, 1986

2000s, Progressive magazine interview (2003)

Nationally syndicated column number 219, Rogers Gets Six Shiny Dimes From Oil King (1927). <ref name=columns2>
Weekly columns
Variant: I never met a man that I didn't like.

Laughing Gas (1936)