Joshua, 32, was turned over in his first meetings with both men, with Ruiz knocking him out in their Madison Square Garden showdown in June 2019.
Slick southpaw Usyk schooled AJ in their Tottenham tussle last September, but the Brit believes the retired Wladimir Klitschko was the toughest nut to crack.
He recalled: “The hardest opponent I have come up against would be Wladimir Klitschko – definitely.
“The passing of the guard. The young lion versus the old lion. At the time I fought him, I thought it was definitely too early but it was his last fight.
‘So if I didn’t fight him then, it would have been too late. It was risk versus reward and I thought boxing needed it.”
Joshua was knocked down in the sixth round of his 2017 Wembley war with Klitschko but rallied to score an 11th round stoppage.
He said of the fight: “Sometimes, due to a lack of experience, we make it harder than it sometimes needs to be.
“He had definitely more knockouts on his record than I have fights and knockouts combined. He is experienced and very strong and it was a tough fight.
“Before that stage, I was knocking guys out within six rounds and seven rounds.
“Wladimir took me 11 rounds – somewhere I had never been before. He was my toughest, for sure.”
Joshua is currently gearing up for a high-stakes rematch with Usyk – which is set to land in Saudi Arabia.
July 23 is the slated date for the pair’s desert dust-up, but promoter Eddie Hearn recently revealed it could be pushed back if contracts weren’t signed by the end of the week.
He said: “There is not a problem now but if it is not announced next week then it could be put back a week or two.
“The fight is definitely happening, the 23rd is the date that everyone has agreed on and everyone is fine with. If we agree next week then everything is fine.
“The date of the fight and the time to train are not the problems, it is more about the extra obligations of TV, press conferences and promotion.
“Eight weeks out feels OK but once you get down to seven and six, it’s a problem.
“Both fighters know that if we have to do a media tour then it needs to happen in the next 10-14 days. Beyond that we will have to move it back a week or two.
“If we go July 23 then we are nine weeks away and we need to announce next week, it has to be next week.
“They will want us to do a media tour, we would have to go to the Middle East and London, maybe even New York, so we really need to announce next week.”