Whenever he does that Spin Puch and just kept spinning for about 10 seconds, he usually gets me every time. I have beaten him, but it seems he is a bit harder to beat than Mike Tyson.
SMM: Other than his Super-Spin-Punch (which is pretty easy to avoid), he's no tougher than Glass Joe. It just requires a lot more hits to knock him down. His only other punches can be telegraphed seconds before he actually executes them.
Tyson: The opening 90 seconds where he throws dynamite uppercuts is hell. If (big if) you can get through that being knocked down 0-1 times, you have a good shot at winning, since the rest of his punches aren't anywhere near as devistating. However, that 90 seconds is the hardest in the game, so Tyson wins based on that.
I beat SMM in 2:03 in round 1 once. You can get 1-2 stars from him every time after he gets up by hitting him with timely punches to the face, but don't do it when he goes back to wind up for the big Spin Punches. And if you time it perfectly, I think you can uppercut him as he's coming down with the spin punches but it's foolish to try.
I've done that with Bald Bull before when he does his Bull Charge, but it's kinda hard to get the timing right.
And it's impossible to get any uppercut stars on him in round 1, unless you use a Game Genie code, if there even is one.
But since only Super Macho Man and Mike Tyson are the only two boxers who can knock you down in one punch with full energy, well...I guess that'd make them the strongest, not the toughest. I think Sandman is "tougher" since it's impossible to stun him and pick up a Star for the uppercut in the first round.
Out of those 3 I'd rate Super Macho the easiest, he's the only one I can TKO in round 1.
I'd rate the 1st Don Flamenco the easiest fight in the entire game. All you do is punch so he punches, then continue the left-right combo 'till he's down. Then when he gets up he immediately throws his uppercut (he has little energy left), but you just left-right combo him again and he's KO'ed. I think it's usually over after 35 seconds if done this way.
I've never really had much problems with Super Macho Man other than his Super Spin Punches. The reason why he got me was only that I was either dodging too early or that I was dodging in the wrong direction. I think it was a little bit of both though dodging in the wrong direction does appear to be my dominant reason. But anyways, once I've mastered that (dodging right which is the correct direction) I had no other problems.
Super Mach Man was easier than Mike Tyson and Mr.Sandman for me of the most.
Super Macho Man has some unstoppable pecs, clever one-liners, and roid rage but he's still easier to defeat than Tyson. Mike's style is impetuous, his defense is impregnable, he wants your heart and he's going to eat your children. Of course he's harder...
As other posters have mentioned, Macho can be dropped in the first round with near flawless timing. Tyson is manageable after that initial "uppercut of death" salvo, but I found him to be one of the tougher final bosses in any NES game I played considering he will absolutely obliterate you many times before you finally best him.