no.. he isn't..
every sport has their own league, i'm not very educated in many sports other than tennis but Nadal is good at tennis yet but tennis is a 1-on-1 sport, what about team sports? like Messi in Argentina - football, rugby, American football, etc... or watersports, cycling, baseball...? i don't think it's possible to narrow something so general down to one man.. that's like saying Albert Einstein was the best scientist of all time, he was one of the most influential but to say he was the best is just ignorant because there are/were many scientists covering different aspects of life..