Totally subjective, like huaxiong said above.
If you want to drag previous generations into it, though, to boost Japan's "roster", you have to also consider that the vast majority of great Japanese games, especially RPGs, got their roots in western games like Ultima (Final Fantasy), King's Quest (most JRPGs of the past generation were fundamentally adventure games along the lines of KQ, with strategic combat elements from games like Ultima), etc.
I would personally have to favor the US as being the most prolific, as far as new game ideas and IPs go, but largely that's due to size, not raw skill. If I was judging a nation's game dev talent by raw skill over, say, game dev population... I might have to go with the UK. Japan makes good stuff too, of course, but their skill lies primarily on the creative/storytelling side of gaming (which many would argue is the most important), whereas the US clearly leads the world in regards to gaming technology, and presentation ability. You could argue that Western game storytelling is much more mature than Japanese game storytelling, too -- but having a "grown up" story doesn't really make a game better or worse than one with a story suited more for younger folks.
Six of one. Half dozen of the other.
