The OSM tiles would work well for that. The one caveat is that if you are expecting a million users, then that will put a large load on the OSM servers trying to serve all the image tiles needed. You could probably set up your own tile server, and create and store tiles locally to be served to your game.
An alternative to setting up your own tile server is to use the MapQuest tiles - just let them know of your estimated usage level:
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