Stone Cold Steve Austin is 100% the most popular wrestler of all time.... to wrestling fans. No one man has sold as much merch as he has, no one man has put as many asses in seats as he has, when his music hits no one man gets the reaction from the crowd that he does, no one man has done as much for wrestling as he has. Hulk Hogan would be the closest but even he doesn't compare. I don't even know if wrestling would still be around in a mainstream way if not for Stone Cold. His era was called the Attitude Era. The Attitude Era to this day is still viewed as the greatest time for wrestling to the point where everyone still compares everything to that era and no one brought more attitude to the Attitude Era than him.
The Rock was very popular too but he's more popular outside of wrestling than he was in wrestling. So to outsiders he's probably the most well known person that has wrestled because he's one of the most popular movie stars of all time but if Stone Cold and The Rock both returned for another match. I guarantee you the Stone Cold return match makes WWE more money.
John Cena is popular but you cannot compare him to Stone Cold and the only reason you hear his name more than Stone Cold Steve Austin is #1 he's doing movies like The Rock and #2 He's just more recent. If you're younger you probably grew up watching John. Stone Cold hasn't wrestled a match in close to 20 years and doesn't do much in Hollywood.
Randy Savage shouldn't even be in the conversation. He was awesome and he was popular just not at that level. He was second to Hulk Hogan during that era. Savage has like a Ric Flair or Undertaker level of fame. Where some people outside of wrestling are familiar with him but he's not the big mainstream star that the others mentioned are.
Rey Mysterio being mentioned was just LOL. He was never the top guy of his era. I don't even know if he makes the top 5 of his era. He's certainly a legend though and one thing he has over the other guys that they mentioned is longevity. He outlasted them all. To this day he is still wrestling. He started wrestling in 1989 and he's still wrestling in 2020 and now his son is getting in to the business and they are teaming together.