The striker missed eight games with a calf injury that he suffered in December, but Aguero has since come off the bench in City's last two games to score in both, including in their 4-2 win over Cardiff City at the Etihad Stadium on Saturday.
"I'm really happy to see him back," Zabaleta told reporters. "He needs to get back to his best in a physical way because he's been out for a month, but he took 30 seconds (actually 50) to score a goal when he came back against Blackburn and, on Saturday, 20 minutes.
"It's so easy for him. Every time we talk about good strikers, Sergio is one of the best in the world.
"Probably [Lionel] Messi and [Cristiano] Ronaldo at the moment are in a different world, but I think Sergio is one of the best and we are very lucky to have him in the team."
Aguero has 21 goals to his name this season.