The 43-year-old German, who will retire at the end of the season, has won seven world titles and 91 races and is the most successful driver in the history of the sport.
"His record will, I think, remain unbroken forever," Stewart told BBC Sport. "He's one of the greatest drivers of all time, although it's not possible to say he's the greatest.
"The number of Grand Prix he's driven are considerably more than anyone else. Juan Manuel Fangio didn't come to Europe to race until he was 39, yet won five world championships. His ratio of wins to races was extraordinarily good.
"Jim Clarke likewise. Myself, I only raced in 99 Grand Prix races but won 27. Although that's nothing like the 91 that Michael has won, the average was good."
Stewart added that Schumacher also had an advantage of racing with the best car on the grid during his time with Ferrari.
"Michael's record in F1, particularly his Ferrari period, was so dominant because the car and engine was unquestionably the best on the grid and there was no doubt about who was number one and got most attention in the team," he said.
Stewart won Formula 1 world titles in 1969, 1971 and 1973.