I have an iPhone so I am going to be as unbiased as possible here;
But I believe McDonalds would be there best option as it is an international company. Like literally everywhere.
Starbucks, for example, is in only a small few locations across the world, and here in Australia could be counted on one hand... two hands tops. Walmart isn't anywhere near as wide-spread as Starbucks & 7-Eleven is on about the same page.
On that note too, at least I can only confirm in Australia, McDonalds here has free WiFi in every store. I can't confirm that worldwide as I haven't had the luxury of travelling; but McDonalds are universal
But remember, I have an iPhone so you can disregard my opinion if you wish