I would not want to pick any small slick heavyweight over Frazier. I think he would eat them up moreso than Marciano. If we must pick one however Tunney has as good a chance as any.
Greb had 28 fights after meeting Tunney for the last time ,he won 26 of them including wins over Johnny Wison,Mickey Walker a no dec with Rosenbloom ,no10 LH,a win over Allentown Joe Gansno 5 at M,a win over Tony Marullo ,no12 at LH .|A win over Art Weigand ,no11 at M,a win over Jimmy Delaney no8 at LHand a win over Romero Rojas no 4 at H,that isnt a bad decline!Its just possible that the tide turned during the Greb Tunney fights because Tunney got bigger and stronger,and improved as a fighter ,not because of Grebs decline.Greb himself said" let someone else fight Gene ,the sonof a ***** is getting too big and too tough."Greb was what 32 when he died hardly finished.
Well it's like this: if you have a great fighter, and he loses say 30% of his ability, you are still left with one hell of a fighter. Joe Louis was on a ten fight win streak including over contenders like Brion, Savold and Bivins until he met Marciano, but does that mean he was a great as he used to be? No, but he was still a damn good fighter. Ali, after being outlawed from the ring collected wins over Quarry, Bonavena, Frazier and Foreman, does that mean he was as good as he ever was? Greb obviously still had a lot left, but not enough to deal with a marvelous entity like Tunney. I do agree that Tunney's development was a factor as well. But what does that tell us? He couldn't get past Greb when they he had a slight weight advantage. Against Frazier he has a slight weight disadvantage.
If you look at my pick you will see I went for Frazier,just didnt want Tunneys wins over Greb to be thought of as victories over an over the hill fighter,and that is what badly declining means to me,losing 2 out of 28 ,against rated fighters ,from Middle to heavy doesnt seem like a bad decline to me,Greb had weight problems against Walker,and had no food for two days before their fight,,but his ability was still there,there was probably financial reasons he killed himself to make Middle,the Lhvs werent very marketable.I dont discount your thesis entirely ,I beleive Moore and Walcott were in decline when they met Marciano,and gave such great efforts because though not their best ,they still had some magic dust,so I agree to some extent ,it was just the phrase "badly declining".
I dont think Greb's relevant. It's seem like a long-way around a fairly straightforward question. We have Joe Frazier on film. We have Gene Tunney on film against a few fighters (inc.the aggressive Jack Dempsey and Tom Heeney, a 200 fighter.). We dont have any footage of any of the Tunney-Greb fights, and no footage of Greb at all, who's style was considered particuilarly unorthodox and unique. Greb's a completely obscured path to go down. My GUESS is that Dempsey would be a better stylistic match to Frazier than Greb would. I think Tunney stands a great chance against Frazier just going on what I can see on film. Good boxer, good footwork, durable, nice jab and strong uppercut, he'd give Frazier a good fight.