Aguero, 31, scored a hat-trick to become the highest overseas goalscorer in Premier League history as City thrashed Aston Villa 6-1.
"He is a legend," added Guardiola.
Aguero's contract ends in 2021 but Guardiola has not given up hope the striker, who joined from Atletico Madrid in July 2011 for £40m, will sign a new deal.
"It depends on him, it is his decision," said Guardiola.
"This kind of player... there are many in the incredible history of English football. He helps to make the Premier League better."
He added: "The best is Messi.
"Messi is number 9, 10, 11, 6, 4. But the rest is Sergio," said the City boss.