The fact that Willard was the undisputed champion, is itself enough to make him a more successful fighter than Ruiz. If you go by names, Ruiz does actually have some good names on his resume, though it starts to unravel when you look at the circumstances. His best result was probably beating Ramhan. Regardless of who was better, I don't think that a win over Ruiz is likely to translate into a win over Willard.
Roy by wide decision. He'd be cautious and pick his shots, and Willard never saw speed like that. Size and strength counts for nothing if there is nothing to hit, and I can't see Jess landing anything really solid on Jones. Jones wasn't the proverbial 98 pound weakling when he fought Ruiz either, and I think he hits hard enough-and certainly fast enough-to gain Willard's respect. Smaller Jones may be, but in terms of skills they are in different realms.
You seen the footage of Willards losses pre Dempsey? Jones wasn't a fast HW. Jess Willard is more mobile than RJJ at HW.
Nothing to do with the fact that Vitally Klitschko was likely to be his mandatory? Was there nobody worth fighting at heavyweight?
Dempsey is 2X faster than HW RJJ. Check out the footage. Dempsey circles the ring like it's nothing, with very quick footwork. Different leagues. RJJ at HW is not that fast. Bob Pastor is faster. He has good sharp movements, but overall his speed is not giving him comfortable control over Ruiz. Dempseys speed had Willard doing the Wooden Indian.
This a 12 rounder in modern times where Jess gets access to Roy's special "vitamins", or in Willard's era with a 200 round limit, brandy and strychnine in the corner for sippin' between rounds, and a diet revolving around pone and fat back?
Wlad and Lewis are cream of the crop. Jones showed when he beat Ruiz that old time heavies could be competitive today. Ruiz more represents the so-called modern heavy. The Wlad's and Lewises don't represent the average, they are the top of the line. I'd wager that JQ, Charles, Walcott, Machen, Zora Folley, Floyd Patterson, Rocky Marciano, Archie Moore etc would all have reasonable chances (some of these may be favored) to beat Ruiz and that level of modern day heavyweight.