Safest bet is...
All time: Pelé/Maradona/Ali. Those three are from the pre Internet era (ie infinitely less exposure) and even today whoever is a true sports fan know who they are.
Currently: Cristiano Ronaldo/Lionel Messi. Football is by a huge margin the most popular sport across the world, I just can't comprehend how anyone can even try to dismiss this to name some player for other much more unknown sport (again, in a global scale) like basketball, eggball or hell... even tennis.
Also, MEGALOL to anyone naming Jordan because the Nike clothing. By the very same logic, because of Converse, Chuck Taylor should be even more famous than Jordan, you can bet it's not the case.