I absolutely don't doubt that Riddick could defeat Joe, I lean towards Frazier for a couple of reasons. Bowe was great at mid range but never faced a truly elite relentless pressure fighter like Joe Frazier. When Bowe was crowded he struggled defensively , squared up and got hit clean more often.
I'm not sure Bowe could take those left hooks to the body and liver for 15 rounds, and I'm not sure Bowe hits hard enough to stop Joe before he gets dragged into deep water and drowned. Frazier by 13th round stoppage.
I think Bowe could floor Joe early on but the latter really turns it on around the 8th round and Bowe is more than a bit overwhelmed at the end. Majority decision 12 Frazier, but Frazier stops him in 14 if it's set for 15. Bowe got hit WAY too much.
I think Joe Frazier would have struggled against someone whose was also incredibly powerful but had a great uppercut and inside game - Riddick Bowe for example. People don't realise just how skilled Riddick Bowe was as well as powerful. Bowe's stamina and conditioning was outstanding in the first Holyfield fight, he was throwing brutal combos when the bell sounded.