#1 David Beckham
Let's not look at the on-pitch side of thing - David Beckham was, is, and probably will always remain the MLS's greatest ever superstar. He will be the man who changed the image of the league forever.
In the summer of 2007, when Beckham moved from Real Madrid to Los Angeles, a precedent was set. The MLS had become an attractive destination for even superstars of Beckham's profile.
On the pitch, too, he didn't fare too bad. He won two MLS Cups title for the Galaxy and still remains a kind of father figure at the club - who unveiled a statue in his honor last month.
Beckham did move to Europe on several loan stints during his time with the Galaxy, but he was always present for the Galaxy during the MLS, ensuring his loan stints came largely during the off-season and the very start of the MLS regular season.
If anyone can beat the impact that Beckham has had on football in the United States, he'd have to achieve something incredibly special, because that is the nature of the image that Beckham currently holds.