The 50-million-pound midfielder, Villa's record signing from Everton, revealed that Emery has taken him out of his comfort zone, forcing him to develop new facets of his game.
Onana credits the manager's intense approach for inspiring the squad as they aim for success on multiple fronts.
"I've never seen anyone as obsessed with football as he (Emery) is," he told Sports Illustrated.
"I think his entire life is just about football. Sometimes we'll fly back from European games, when it's probably 2am and he'll probably be in his office trying to rewatch the game. I find that fascinating, but I think that makes him the manager he is.
"He's made me uncomfortable. That's how you grow faster as a football player, but also as a man.
"I've had many managers before. Unai Emery is the best one I've ever had.
"I could feel in the conversations we had that this guy could take me to the next level. I knew what he was expecting of me and what he could bring to my game that could bring me to where I want to be.
"I spoke to many other managers at the time. This was just the right choice to do at the time. Still is today, by the way.
"We have a great group of human beings, good atmosphere, the fans, the stadium and we have a genius as a head coach. I think that is pretty good."
Aston Villa are targeting glory not just in the Premier League but also in European competitions, with Emery's relentless standards seen as a key factor in the club's ambitious push this season.