In their primes, Mays hands down. The only problem is that Mays hit the wall at a much younger age than Aaron. Aaron was able to stay productive for a few years longer than Mays did so he eclipsed him in stats. That said, you have to take into account defense, home ballpark, and the fact that Mays missed almost a full year in the military. When all is said and done, I'll take Mays, but not by too much.
I will say that Aaron was much better than Clemente. I feel that Clemente is overrated these days a little bit. Yes, he was one of the most graceful players ever, and possibly had one of the best RF arms as well. However, not many know that he only drove in 100 runs twice in his career. He was a hell of a hitter, but I'd say he was more comparable to Tony Oliva than Hank Aaron at the plate.