I never met a man I didn’t like.

Will Rogers (1879-1935) American humorist

Motto
    (Source)

Rogers' first use of the phrase in writing comes from "Letters of a Self-Made Diplomat to His President," Saturday Evening Post (1926-11-06):

I bet you if I had met him and had a chat with him, I would have found him a very interesting and human fellow, for I never yet met a man that I dident like. When you meet people, no matter what opinion you might have formed about them beforehand, why, after you meet them and see their angle and their personality, why, you can see a lot of good in all of them.

(Misspelling of "didn't" in the original.) Rogers was writing of his regrets over not having met Leon Trotsky while visiting the Soviet Union. The article was incorporated into a book Rogers published about the trip, "There's Not a Bathing Suit in Russia & Other Bare Facts, ch. 4 (1927).

Two other early references, the first from his "Weekly Article" column (1930-06-29):

You know I have often said in answer to inquiries as to how I got away with kidding some of our public men, that it was because I liked all of them personally, and that if there was no malice in your heart there could be none in your "Gags," and I have always said I never met a man I didn't like.

And from a speech at a Boston church, the same month:

I’ve got my epitaph all worked out. When I’m tucked away in the old graveyard west of Oologah [Oklahoma], I hope they will cut this epitaph -- or whatever you call them signs they put over gravestones -- on it, 'Here lies Will Rogers. He joked about every prominent man in his time, but he never met a man he didn’t like.'

That reference was picked up in AP news stories, and Rogers used the phrase for the rest of his life.

More info on Rogers' motto and the above quotations: here, here, and here.