ok. I noticed that a few of you have thrown the name of IPL in, but aren't really sure about the details. I thought I could help.

I am an Indian residing in the US. I follow the NFL, NBA, IPL, EPL and know a little bit about most of the other popular leagues that have been described here.

A league should be judged for popularity based on the number of eyeballs it gets and the number of countries it represents (as someone pointed out, having international players would bring in a new audience who enjoy watching their compatriots and thus get hooked).
I think, the order of popularity of the leagues would be as follows.

Most popular leagues
1. EPL
2. NBA
3. IPL

The most popular has to be the EPL or the NBA, given that they both have international players from some 40 countries every season and also because of the brand name they have established. The IPL is only in its 5th season and has risen phenomenally. It has players from some 15 countries and is followed in a lot of other countries in Asia and Africa. You have to take my word on the IPL. There's 1.2 billion Indians and you would expect atleast 35-40% of them to follow the league. And then there's Pakistan, Bangladesh, England, South Africa, Australia and a host of other smaller countries.

If you take money into account, the order is as follows. This can be verified through many fact based lists posted online

Richest leagues
1. NFL
2. MLB
3. NBA
4. EPL
5. IPL