The ex-Tottenham boss also revealed that he wanted to sign the 27-year-old playmaker when he was in charge at White Hart Lane, but was unable to finalise a deal.
“Cazorla is a superb footballer and I think Arsenal have got a real diamond,” Redknapp wrote in The Sun.
“However much they got for Cesc Fabregas, £40 million or whatever it was, they have managed to replace him for less than half the money.
|9/2||Santi Cazorla is 9/2 with William Hill to score first in Arsenal v Olympiakos
“Probably the best one any side made this summer, in my opinion.”
Cazorla was brought into the Emirates with Robin van Persie facing an exit. The Dutchman eventually moved to Manchester United and Redknapp believes Van Persie could be a difference-maker in this season's title race.
He added: “If you had to put your life on one man putting the ball in the net, you’d be hard pressed to come up with someone better — and he will be worth another eight, nine or 10 points to United this season.
“In my book, that could be the difference between winning the title and not.”
The 65-year-old also hailed Tottenham's new-boy Jan Vertonghen, who arrived at White Hart Lane shortly after Redknapp's departure.
“He’s a hell of a player — one of the growing number of Belgians who are tearing up the Premier League,” he said.
“One of the things which has really impressed me about Vertonghen is his mentality — he is such a strong character.
“He’s an established international and is already proving that he’ll have no trouble adapting to the English game. Even if he had cost £12m, he would be a real steal.”