"Physcially better" is very subjective. Mike was more of a genetic outlier, full adult development at a very young age. Mike at 18 looks like he could have been the dad of Ali at 18. He was a bundle of 218 lbs lean fast twitch muscle, with extreme speed and power. Ali was 6'3" with great coordination and reflexes. So who is "better" is take your pick based on personal preference.
Mike matured earlier. But I am also sure he was on PEDs. As boxers, they are very different, but none of them was something exceptional physically. Which doesn't mean they were not great boxers (including the athletic aspect of that) - they were, but that's different, as boxing is not one-dimensional discipline.
Before I answer my response has nothing to do with who's better, or who would beat who.......simply who was the greater specimen. Question being "Physical Specimen" the answer is Tyson. He physically was a freak of nature. Ali was indeed physically gifted...............Iron Mike was an absolute freak of nature that you really can't duplicate. He's like Wolverine being created in the laboratory freaky. Mike was really a LHW by frame, he shouldn't have even been able to be a HW. It's really crazy when you sit back and think about it.
Both were extraordinary in their own ways. Ali was super quick of hand and foot, great balance and poise. But what took him to another level was his ability to absorb pain and punishment. His chin was excellent but it was more than just that…his powers of recovery were superb. Of course some of it was mental strength, but mental strength can only take you to a certain level if you don’t have the physical gifts to bolster that inner belief. Vice versa too of course. And as for Tyson…what can you possibly add to a 5ft 10, bull necked lump of granite that’s as explosive as dynamite, moves like a black mamba and hits like a rhino. With that build, he had no right being as explosive or as sublimely quick as he was. Strong yes, but as athletic as he was? Incredible. His lack of height and reach couldn’t stop him achieving what he did due to that immense physicality of his. All in all on a purely physical basis I’d give it to Tyson. Where Ali takes it is the combination of his physical and mental strength.
There are some great posts so far. I know that Tyson has not come out and admitted PED use. If he has never used PEDS then I believe he is a greater physical specimen. His build and athleticism at such an early age, if all natural, is almost unbelievable. For that reason, he would be the greater specimen.
There are two & logically related mistakes there. 1) Tyson or anyone *could* have been on the juice. But there is no decent evidence he either ever was, OR folks his size or with his history (bulky by his early teens as a poor teen without any training) need to do drugs to get that size. I just posted a video & description of McVey, who started around the turn of the 20th century-decades before PEDs or even any meaningful modern weight training was even invented-with a similar look & build & size to Tyson. And of course being that muscular with only routine work or sport-like McVey or Tyson-is exceptional. Add Tyson's great speed of hand & foot, of course that is the definition of exceptional! NO HW at any time & even considering PEDs had his combination of speed muscle & strength. Ali was an outliar in speed reflexes & coordination, + ability to know & control where he was in space. Innumerable fighters have had the same or greater trainers-None were as unusual as these two. To find a HW as fast as Ali-(maybe less in absolute hand speed), you have to go to a rare dude like Patterson-& he was a rehydrated LHW at his biggest by modern standards. Ali with hisnatural size & agility & speed was freakish, just like Tyson.