Who is the most marketable sport star today? Tiger woods? not now David Beckham? possibly Michael Schumacher? to old
I doubt a traditional sports star would make the top 5. The real answer is probably a kid like Travis Pastrana. He already has two TV shows and countless DVDs. Sponsored by MTV, RedBull, Shoe companies, Clothing lines, etc. He likely will not make as much as a-rod until the networks primetime one of his events and figure out the market. but the trend for these guys is up no doubt.
With MMA gaining popularity by the day, I would say GSP is gaining marketability very fast. He is definitely the most popular sports star in Canada.
I would've said Lebron James if he went to a contending team where he was still the focal point, ie. the Bulls. Next best would be Tom Brady.