before coming into the thread I though "probably jordan or maybe muhammad ali". And... see the results from neogaf say the same.
These are two people whose names will forever, or at least for as long as we live, be synonimous for the sports they played. However, since Ali's career has been over for so much longer than Jordan's and because basketball is a game that is more famous than boxing all over the world (it's number 2 sports in many many countries. Nearly nowhere number 1 but number 2 is enough to know about it - and about Jordan), Jordan comes first nowadays.
Question is how to interpret the question: Is it just about a certain point in time in which one sportsperson was the most famous (known to nearly every living human at that point in time) or is it about a person who transcends through the ages and lives on as a legend (which could be interpreted as "more famous"). But... whatever, we're going semantics here. Doesn't matter.