Probably water, even with the large amount of privatization.
"Today and forever I am your better, Arthas." - Illidan Stormrage
i would probably say the atmosphere
or the sun, yeah
the sun is our largest shared resource
If you are particularly bold, you could use a Shiny Ditto. Do keep in mind though, this will infuriate your opponents due to Ditto's beauty. Please do not use Shiny Ditto. You have been warned.
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I had man made things in mind.