Ruiz and Klitschko are the only two fighters to have so far knocked down Joshua in his 23-fight career.
And the British heavyweight believes it was Ukranian who hit him hardest.
"Who's hit me the hardest? Yeah, Klitschko," Joshua told the Pound for Pound podcast.
"'I was gonna say Ruiz, but when he hit me that was like a concussion on the back of my head."
Joshua and Ruiz's rematch is scheduled for 7 December in Saudi Arabia.
Ruiz's surprise victory was Joshua's first professional defeat, having survived a gruelling fight with Klitschko in 2017.
Their battle went to the 11th round, with Joshua knocked down in round six, and he understood the stakes.
"When I went in with Klitschko, yeah, I just knew this could go two ways.
"I could either win or I'm going to be laid out on my back. You know when you get carried out on a stretcher?"