I had this very same debate about 18 yrs ago with an American partner of mine. He said Ali, I said Pele. Both men had major impacts in argueably the only two truely international sports, football and boxing. One can make a valid arguement for both men.
I just think that Pele's stats is what sets him apart from most other athletes. Most other ballers don't even come close to having the numbers Pele has. On the other hand, just based on stats, other boxers (maybe not heavy weights...correct me if I'm wrong) have done similar things like Ali. Ali's impact on the American political landscape, among other things, is often what sets him apart from others of his era.