I love Zoomah but Corny was a bloody good fighter himself in a pretty damn strong era, depth wise. I've got to go with the exceptionally proven Nelson but Boza is no easy out at any rate.
On paper I would go for Azumah without a doubt but Cornelius fought in a golden age of exceptionally tough competition where every fight was a war. I don't feel his brief championship run really reflects his quality and I think any great SFWs of history would have had their hands full when the contenders were of the quality of Limon, Chacon, Navarrete and Arguello. Put the Boza-Edwards of the Chacon fights or Limon fight in the ring with anyone and you will be faced by a super tough and determined guy who could both box and punch. In fact Boza probably could have relied on his boxing skills more and avoided a lot of punishment but as a fan I am glad he chose to slug! Apart from Navarrete (who was still very good) and an early loss in his tenth fight he only lost to ATGs or HOFs (or soon to be). For me the Navarrete fight was the direct result of his last two fights being fight of the year level wars and I also had him winning the Chacon rematch by a few points. With a slight change of circumstances I could imagine a universe where he could have been a 6 or 7 defense champ which would statistically make this seem more competitive than it may at first appear. Having said all that I still give the edge to The Professor, as his nickname suggests I think he was a more three dimensional thinking fighter with Boza lacking any specific area he would have a significant advantage. I feel everything Boza could do Azumah could do a bit better. The only factor is Cornelius' sheer will to win and guts. Unless Nelson is at his peak it could be a torrid battle with maybe a possibility of an upset if over 15 rounds for Boza-Edwards.
Nelson struggles with Edwards in the 1st meeting but wins a close decision. In the rematch as Nelson often does way better in rematches he stops Edwards late on in a less competitive fight.
Boza -Edwards would make Nelson fight to win. But the better skills, and naturally bigger Nelson would eventually stop Boza. Heck of a fight it would be though.
Watched a good bit of Boza vs. Arguello the other night. Good as Cornelius was, there are levels to this game. And Nelson was on a different level of CBE. He wins by stoppage, perhaps in the corner, after taking control somewhere around the fourth or fifth and closing the show before the 10th.
Boza was like Real Deal and Terry Norris in that they had sublime boxing skills but once hit turned to brawling. Boza exchanges once too often with Azumah and is stopped mid rds.
Nelson was a consumate pro and a man worthy of great respect, but he caught a lot of breaks in his fights. The first Fenech fight, for example, and the first fight with Azabache Martinez. He was far from unbeatable and, in my opinion, there his a very large gap between him and an Arguello or a Chavez. In a fight with Boza-Edwards he would, like everybody else that fought Cornelius, be forced to go to war. He may very well win that war, but it is anything but a foregone conclusion. Each is more than capable of hurting the other, and it is very likely that Azumah will not match the pace that Boza sets.
