It wasn't Adolph Hitler, although that misunderstanding has been propagated endlessly. For a lighthearted and entertaining explication of the answer, spend 3 minutes viewing the QI segment embedded above (skip to 31:30 if it doesn't autostart there)(the link under the embed goes to one that autostarts at the right spot, but for some reason I can't configure my embed to start at 31:30).
Those in a hurry who want the TL;DV answer can find it beneath the fold, but you'll miss the wise crack about the "far right."
It's true that Hitler did not congratulate Owens on his accomplishments, but he did not congratulate other medal winners either. Owens has said that he felt he was snubbed by President Franklin Delano Roosevelt, who did not send a telegram or invite him to the White House.
A German sports reporter, Siegfried Mischner, has claimed Owens carried a photograph of himself shaking hands with Hitler and called it 'one of my most beautiful moments'.More at the Daily Mail.
Mischner, 83, says he and several other reporters saw the handshake behind the stands at the Olympic stadium but never mentioned it. Owens and other eyewitnesses always maintained that the story of Hitler's snub was exaggerated...
'After all those stories about Hitler and his snub, I came back to my native country and I couldn't ride in the front of the bus,' Owens recalled. 'I had to go to the back door. I couldn't live where I wanted. Now what's the difference?'
Owens was given a tickertape parade in New York. But when he arrived at the Waldorf Astoria hotel for a reception in his honour, he was instructed to take the service lift rather than the normal guest lift, which was reserved for whites.
President Franklin Roosevelt never congratulated Owens or invited him to the White House. 'Hitler didn't snub me - it was FDR who snubbed me,' Owens said.
And Owens had his own memories of Berlin which differed starkly from the propaganda version. While the Nazis vilified the black American athletes, the German people cheered on Owens and his team-mates, clamouring for photos and autographs.