Interesting. You guys have got me thinking with these different answers. I would have thought that striker fired handguns were the most common platform. But now I think that they are the most popular if you count only the last decade or so.
I would think that the vast majority of guns being sold these days are handguns, and of those, most would be 9mm striker fired. But if you figure in all of the areas in the US where handguns are severely restricted but long guns (particularly shotguns) have little to no restrictions, I would say that shotguns are a good candidate for being the most common platform.
It sounds like there are a huge number of ways we could break this down - by caliber, by the shooting mechanism (striker vs hammer), by long gun vs hand gun, etc etc.