City fell 1-0 down to their recent bogey team before Argentine Aguero equalised and he struck again to complete the rout with his 14th league goal of the season.
"He is just 26 years old and he can improve every day," Pellegrini told reporters. I think he is in the five top players in the world. He is scoring but he is also playing well."
Pellegrini also hailed the performances of midfielders Yaya Toure and Samir Nasri after champions City consolidated their position in second place in the table, six points behind leaders Chelsea.
"Yaya played very well. Nasri after his injury gave a very high performance. All the team is having more trust but we must continue to do it," Pellegrini said.
Sunderland manager Gus Poyet was philosophical after the defeat, only the north-east club's second in seven games against Manchester City.
"I can't have too many complaints - they were better than us in every aspect of the game," Poyet said.
"We started well, but then Aguero appears from nowhere and does something special.
"Sometimes you cannot complain too much - they were quicker and they were better technically."
(Reporting by Ed Osmond, editing by Greg Stutchbury)
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