Ali would use Tua's head as a speed bag. Ali would cut him to ribbons. Tua does not have the skill, speed or will to win to compete at any level with Ali. Complete and utter no contest.
It's not really fair to compare the two. I don't like Tua's chances against about half of Ali's oppenents.
In years to come people will look at the way Tua is talked about on this forum, and compare it to the way people used to talk about George Godfrey and Cleveland Williams. They will look at his resume, look at the people he beat and their standing in the division at the time, and they will say “what the hell were they thinking?” The reality is that he want even one of the best and brightest also ran’s of his era. Somebody like Ray Mercer would make a far more credible opponent, for the kind of fighters that Tua is routinely matched against in fantasy fights, but you will seldom see his name mentioned in such a context. Perhaps there is something so intrinsically appeal about Tua’s style, that people want to believe that he was something that he was not.
Will those future fans have access to youtube or will they be forced to rely upon boxrec and Ring rankings in evaluating fighters?