I've argued for a while that iTunes was the tip of Apple's sword. Being able to take payments and sell content in more countries than anyone else let them push the App Store farther, faster than any of their competition. Now, years later, no one else is even coming close, and when viewed as an ecosystem play, there still isn't any viable competition. To make matters worse, Apple's model allows them to sell the iPad (and soon, the iPad mini in 90+ countries. And, because they make their money off hardware, they can even sell it where there's still no or limited iTunes support. That lets them seed the market so hardware is in hand when iTunes eventually, and inevitably follows.
By contrast, Amazon's content appliance strategy with its heavily discounted hardware doesn't allow them to sell the Kindle Fire line in any country without an Amazon content store to subsidize it. They literally can't afford to. That means no seeding, and since Amazon has been nowhere nearly as successful as Apple in pushing their content store internationally, very limited distribution.
Talking about it is one thing though. To really appreciate the difference, you have to see it. That's where Graham Spencer of MacStories comes in. He's taken the time to map the media ecosystems of Apple, Microsoft, Google, and Amazon, and as they say -- a picture is worth a thousand words. (Or in this case, really shows the cost vs. value prop for an iPad over everything else in the market right now.)
Whichever company is the “winner” depends on your circumstances (location, device, etc), but if you were to generally draw a conclusion I think it is clear that Apple would lead, Microsoft would be second, Google third and Amazon fourth. Again, that is a gross generalisation of all the above data but ignoring the exceptions (such as Amazon’s eBook dominance), I think it’s fairly evident which companies have put the most emphasis on international availability.
I'd even argue Amazon's eBook "dominance", because like Serenity Caldwell pointed out at Cingleton deux, all of Amazon's early ebook efforts were little more than black and white scans of black and white books. They were and are the volume leader, but until recently they didn't even compete in the quality game. For a certain segment of the market, better is better.
Check out Graham's magnificent graphics and thorough breakdown of media by geography via the link below, and factor that into any tablet or tablet-like-device purchase in your future. Absent content, they're little more than paper weights.