Barcelona star Messi and Real Madrid's Cristiano Ronaldo continue to raise eyebrows with their incredible goalscoring records in recent times.
England goalkeeper Hart, however, has seen enough of the world's best strikers to class Aguero as the most impressive finisher in the game.
He said: "From working with him [Aguero] on a daily basis and from what I've seen this season, Aguero just knows how to score.
"In training he particularly excites you most days but he's also got that ability on the day to do it. We've got a good team, but a long clearance could turn into a goal because he's got that ability to take it round five people and score.
"It's a great thing to have. Sergio is a very natural player and in a situation one-on-one there is nothing complicated about it. He takes that option to score.
"Messi is absolutely awesome, he is a phenomenal player, but I'm backing Sergio."
