In Italy Juventus are king, with a large following all over the country. Inter follow suite, strangely enough, followed by Milan. I say strangely because Milan are a more successful, glamorous club. In the world it is hard to say. I believe Juve is still number one, but I'm sure Milan and Inter give them a run for their money. Milan is huge in eastern europe and asia (China specifically). Inter is pretty big in Sud Americana due to their south american imports like Zamorano, Ronaldo, Javier Zanetti, Recoba, Cordoba etc. . .
I wouldn't think it strange that Inter are slightly more popular, because Milan have only really been more successful or glamorous in recent years. The two clubs were more or less even in the 60's, but in the 70's and early 80's Inter remained competitive while Milan went through a dark age and nearly ended up bankrupt. Inter's slightly greater support probably owes itself to that time period. And, by the way, supporting both Real Madrid and Barcelona is like siding with both Greece and Turkey over Cyprus or India and Pakistan over Jammu-Kashmir. It can't be done, so pick a side.
Who you talkin' too, punk?! I always forget Milan went through a bad patch in the 80's, but even then you'd have to be a moron or a REALLY dedicated fan to support Inter. They're crap.
AC Milan gained alot of international fans because of its successful European run in the late 80's and early 90's. Gulit was the icon or the "Beckham" of the era althrough Van Basten is perhaps the better player.
He was talking to the poster who started this thread, who originally had Real Madrid and Barcelona on his favorite teams list.