The Canadian captured the UFC middleweight belt by choking Bisping out for the count in the main event of UFC 271 back in November 2017 before retiring at the age of 37.
St-Pierre, 40, controlled the contest for the most part from top position but admitted Bisping, 42, was his toughest adversary.
But although he conceded Bisping was more durable than he had been expecting, GSP insisted the former middleweight champion wasn't his most difficult test.
“My toughest opponent was Michael Bisping and the guy that give me the toughest fight was BJ Penn in our first fight," St-Pierre told Fighters Only.
“The guys I think that are the toughest, now? Well, there are a few.
"[Alexander] Volkanovski, [Kamaru] Usman and [Brandon] Moreno are the top three fighters’ pound for pound.”
MMA - or to give it its full name 'mixed martial arts' - has been one of the fastest-growing sports in the world for the better part of the last two decades since the turn of the century.
An early pioneer of the sport, St-Pierre is looking forward to seeing how it continues to evolve over the coming years.
“In the last five years the fighters are better than they ever were before,” he added.
“The fighters of tomorrow will be even better so the best fighter of all time is not even born yet.
"The sport will still improve because of technology and knowledge that continues to make a huge difference.
"Like Isaac Newton says, we stand on shoulders of giants, so the previous generation can learn from their predecessors. This means you don’t do the same mistakes as time goes by.”
And while it's highly unlikely fans will get the chance to watch him compete again, St-Pierre says he takes great pride in staying fit and healthy.
“I’ve always trained because I like to be fit and it’s my therapy,” he explained.
“I’ve done it all my life, since I was a kid, and it’s just a routine that I do that helps me feel better in my own skin.
"I don’t believe training will help you to be a better fighter. However, it will help with longevity and injury.
"It will also help you to look good, which makes you feel good and that can make you comfortable in your own skin and make you healthier.
"If you drive a car you need to give it a service and the same thing goes for your body in terms of strength and conditioning.
"It’s a service on your body at one point you will hurt yourself and get an injury.”News Now - Sport News