I've seen some mistaken assumptions on this subject, so I thought I'd print a list that was in a recent trade mag called RCR Wireless. But it's copyrighted so I'll just allude to it by saying...

By subscribers, the top two are in China and lead by a long, long way. #3 is in Japan and Verizon U.S. is number 4, Telecom Italia Mobile is 5 followed closely by T-Mobile Duetscheland. 18, 19 and 20 are in the U.K. The biggest surprise to me was Turkcell in Turkey with the 14th spot at 17.2 million subscribers.

In terms of providers who have multinational coverage, Vodaphone is the biggest, mostly due to it's partnership in China as well as many other countries.