Manuel Pellegrini's side have opened the season with wins against Newcastle and Hull City Tigers either side of a 3-2 defeat to Cardiff City, as they look to improve on last season's second-place finish.
City have had a busy summer, featuring the signings of Fernandinho, Jesus Navas, Stevan Jovetic, Alvaro Negredo and Martin Demichelis, which Aguero feels puts them on a level above their competition.
Man City's next five matches
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"United will be our biggest rivals, even if they have changed coach, and then Chelsea with the return of [Jose] Mourinho. Tottenham have built a great squad and then there is always Liverpool.
"City have invested a lot for some years and is growing a lot and quickly. We know that we have to win and the new arrivals know that City wants to become one of the big teams."
The 25-year-old also revealed that he will be playing a different position to last season, with Pellegrini keen him to use further forward.
"Sometimes, with [previous manager Roberto] Mancini, I played more on the flanks and more behind the main striker," Aguero explained. "Roberto asked me to move around the box.
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The Argentine also spoke of his delight at seeing former Manchester City team-mates Mario Balotelli and Carlos Tevez thriving following their moves back to Serie A in 2013.
"I am happy for him that he is at a great club like Juventus," Aguero said of his countryman. "It is an honour for Italian football to have someone like him - he will add quality to Serie A."
"Of [Balotelli], I remember only one phrase: 'F*** it!' He was always saying it. We got on well. We respect each other, he is like that, he likes to joke, he is a nice madman."