He also overtook Alan Shearer to become the striker to score the most Premier League hat-tricks with 12.
Richards, who provided the assist for Aguero's very first Premier League goal in a 4-0 win over Swansea, said Aguero is up there with Henry and Shearer as the top three strikers in the league's history.
"When I first trained with Aguero I didn't think he was that good," Richards told BBC Radio 5 Live.
"He didn't train that well, he didn't work that hard and then when he played he scored. He was one of those players that didn't need to turn up in training because he was just so naturally gifted and come game time he was ready.
"To see him do so well has been an incredible achievement for him obviously but great for me to play with calibre of player.
"When I first started at Man City I didn't think I would be playing with the special sort of players.
"He deserves everything he gets not just because what he does on the pitch but off the pitch the most humble person you'll ever meet. For me he's one of the best ever players to grace the Premier League."