Tough to rank fighters from different eras, especially when we get to the Super Heavy generation... In no order, other than Ali:
Ali (everyone knows about ali's skills... arguably the p4p goat, beat bigger guys and his main rivals, in frazier, liston and foreman, 3 of the biggest fights in boxing history... also avenged his defeat to ken norton in his next fight after suffering a broken jaw and same with spinks (minus the jaw) when he was mad old and also has great wins over dudes like patterson, cooper, foster, lyle, shavers, terrell, young, quarry, chuvalo, ellis and mathis)
Foreman (had two great careers, basically two different fighters with two different styles... went from a relentless puncher with amazing power, then came back with almost philly shell defence and picked his shots... though in both careers he was always a clever puncher with a great jab and mad power... also the oldest heavyweight champ of all time)
Louis (beating schmelling in their rematch one of the most important fights of all time and from the footage and fight write ups ive read, one of the best technically offensive fighters in HW history, also one of the best finishers in boxing history too and packed dynamite in both hands... also beat walcott twice which were great wins and holds the record of longest single reign as champion of any boxer in history)
Lewis (really the first of the super heavies that people appreciated more after he retired... great jab and ridiculous power and stamina, beat everyone around him bruno, tyson, holyfield, vitali, briggs, mccall, mercer and avenged his two defeats)