Payet has been sensational for the Hammers this season.
Payet moved to the Boleyn Ground in the summer for £10.7m (via Sky Sports), and has taken little time to adjust to his new surroundings with a string of sensational performances for Slaven Bilic's side.
And Sullivan - who previously owned a controlling stake in Birmingham City with David Gold - believes that the playmaker has shown enough in his first 12 Premier League games to justify being labelled as one of the best purchases he has ever made.
"Payet is the signing of the season. He's the best player I've signed in 25 years. He's a £30m player. He's a supreme footballer. He makes every player in our side play better. On his day, he's world class, he's unstoppable," he told Sky Sports.
Despite not being particularly well-known outside of France before this season, Payet demonstrated on the opening day of the campaign why West Ham parted with an eight-figure sum to sign him, as he was instrumental in the victory over Arsenal at the Emirates.
The 28-year-old - who bagged a staggering 16 assists in Ligue 1 last season - has quickly established himself as the Hammers' talisman with five goals and three assists in the Premier League.
Having been injured since the early part of last month, Payet's absence has seen Bilic's men go on a disappointing run that has seen them drop into mid-table. Since his last appearance, West Ham have gone six games without a victory.
And West Ham's hopes of getting back up the table will very much rest on whether Payet can rediscover the magical form that has led Sullivan to rate him so highly.