that's not a easy question because i have absolutely no clue who is very popular in countries like india or china. maybe the most talked about sports star in the world is someone i never heard about.
but if we talk about a person talked about it most countries, it is a football (soccer) player or maybe tennis player (but unlikely).
lebron james? never...i would not even know who he is if i wouldn't read his name from time to time on vgchartz or a poker forum. except for people who really care about us-sports and the nba, the percentage should be not that high outside of the usa. especially older people have probably never heard this name.
tiger woods is much more realistic, that's someone you can see on tv from time to time and who you can often read about in newspapers.