His two wins over Jack Dempsey, in 1926 and 1927. Nobody else beat Dempsey in his prime.
Preparation as method
Tunney studied Dempsey for years before the fight was made: reviewing film, questioning former opponents, and building a counter to Dempsey's inside pressure based on retreating while punching.
His training routine and reading habits made him an unusual public figure in the sport, and the press treated him as an intellectual, often mockingly.
Philadelphia, 23 September 1926
Tunney outpointed Dempsey over ten rounds in front of a reported crowd above 120,000 at Sesquicentennial Stadium, in heavy rain, to take the world heavyweight title.
The Long Count, 22 September 1927
At Soldier Field in Chicago, before a crowd above 100,000, Dempsey knocked Tunney down in the seventh round.
A rule in force required the scoring fighter to move to a neutral corner before the count began. Dempsey did not do so immediately, and the referee delayed the count by an estimated four to five seconds.
Tunney rose on nine and won the remaining rounds and the decision.
The argument over whether he would have beaten a ten count has never been settled.
The Greb connection
Tunney lost only once as a professional, to Harry Greb in 1922, and beat him in their subsequent meetings.
Retirement
He defended once more, against Tom Heeney in 1928, and retired as undefeated world heavyweight champion at thirty-one.
He married Polly Lauder, an heiress to the Carnegie fortune, lectured on Shakespeare at Yale, and directed physical fitness for the United States Navy during the Second World War. He died in 1978.
Why it goes on a wall
Because he treated a fight as a problem with a solution, solved it, and then left while he still had everything. In this sport that is rarer than any title.
Gene Tunney.