Asked by the Spanish sports newspaper Marca to name the best driver in F1 today, the 75-year-old Scot said: "Mentally, probably Alonso. The fastest is perhaps Lewis Hamilton.
"Rosberg is definitely one of the best, more consistent, and on the same level I think is Vettel. Those are the best," said Stewart.
"In the world there are hundreds of millions of drivers, several thousand make their living from it, a few hundred live very well.
"There are 22 grand prix drivers, maybe six of them are really good, three exceptional, but there is only one genius. So if I had to choose the most complete driver I would say Alonso," he explained.
And not only that, Stewart thinks Alonso, at 33, is in the prime of his racing life.
"I think it's the perfect age," he said. "He has lived the good, the bad and the ugly."
Arguably, however, the present is one of those 'ugly' phases for Alonso; highly paid but missing a third career title ever since he switched to Ferrari in 2010.
"I don't know if he should change teams," Stewart said, "but what I do know is that he will have to make that decision in the next six to eight weeks."