On "Seinfeld," Jerry was the true winner of the Master of His Domain contest.
The contest begins in the Season 4 episode "The Contest," when Jerry, George, Elaine, and Kramer wager money on who can abstain from masturbation the longest. The characters avoid saying the subject directly, using euphemisms such as “master of my domain” and “king of the county.” Their individual temptations and increasingly elaborate strategies drive the episode’s plot.
The original episode leaves the outcome partly indirect. George is later treated as the winner, and that result is reinforced in the Season 5 episode "The Puffy Shirt." However, the series finale revisits the question. While the group believes a plane may be crashing, George admits that he cheated in the contest. Jerry immediately realizes that George’s claimed victory was invalid and declares himself the real winner.
That later revelation is why Jerry is the canonical answer to the question as phrased. George may be remembered as the apparent winner from earlier episodes, but the finale establishes that he did not win fairly. The phrase “master of my domain” became one of the show’s best-known euphemisms.