The title of the thread and the OP ask two very different questions.
If you're talking "AAA" games on home consoles and PC, then of course it's America. The highest-budget, highest-selling games on those platforms are from Infinity Ward, Treyarch, Bethesda, Naughty Dog, Electronic Arts, Bungie -- all American studios.
If you're talking about games in general, I'd argue Japan is superior, although N. America is a close second. Japan has Nintendo EPD, Monolith Soft, Capcom (when it tries), From Software, JAPAN Studio, Platinum, Bandai Namco, Chunsoft, Atlus, etc. North America has Retro, Insomniac, Blizzard, DrinkBox, Monolith, etc.










