IMO there can only be 1 candidate on this thread (given his track record) and that is Rocky Marciano. Not only did he have a height and reach disadvantage, but also a weight disadvantage.
Watching Qawi at 5’6 fighting 6’3 Foreman at HW was fascinating. Qawi was excellent for 6 or so rounds, thudding those overhand rights against big George’s big head. Unfortunately that must have been very demoralising for the little guy, as that big bugger just walked through all of Qawi’s best shots. Anyway Qawi is my pick of them all. Whatever weight he fought, he was giving away many, many inches and still turned out to be a Damn fine fighter.
Starting at #1 on my p4p list and working down, until I find a fighter who was well below average height for his weight, I get as far as #3, Sam Langford.
At least some shorter fighters are blessed with relatively good reaches (see Langford 5’7 1/2 height, 74” reach) - but obviously not all. Being short in both stature and reach would make it really tough. Tyson’s height 5’10”and reach 71”were near enough on par. If Langford’s stats are accurate - then Mike would be dropping 3” in reach even to Sam. Relative to his successes against guys he was giving away at least 6”+ to in height and maybe 13” in reach - Mike has to rank reasonably high in the discussion of best “short” fighters I would think,
Yeah, when you consider the best 3 Fly's of all time (based on fights contested at or around that weight, with no credit for being small at the weight) - Wilde, Canto and Pascal - were all small at the weight (no more than light fly in modern terms, Wilde a minimum weight), it makes me feel they get underrated p4p.