United are expected to come back stronger under Louis van Gaal's management having slumped to a seventh-place finish last term, failing to qualify for Europe in the process.
There were contrasting fortunes for the blue half of Manchester, however, as Manuel Pellegrini's side pipped Liverpool to the title on the final day of the 2013-14 campaign.
Chelsea and Arsenal have welcomed a raft of talent to their ranks during the summer, but Aguero claims that the Red Devils pose more of a threat to City than both London giants.
"This year I think Manchester United will be up there challenging us," he told the club's official website.
"Then you have teams like Chelsea and Arsenal who, as always, will be there or thereabouts battling with us for the league.
"But the team I think who is most worthy of having the best chance to threaten is United, because they are not involved in any other competitions other than the English knock-out cups and the league."