If it's to be an "international" gun, I'd suggest something chambered for a load you can actually find outside the United States. That invalidates .45 ACP, .40 S&W, and .357 Magnum for the most part.
You also must keep in mind that some countries make it illegal to own any firearm chambered in a round used by the military. So, unless you're going to be hitting up the Army depot or local PD for ammo, you might be SOL. So, in certain areas of the world, 9mm & .38spcl will be hard to get.
If you are a super spy, I think a heist of a PD or depot would be feasible, so I'd go with a 9mm. So, I'm either going with a BHP or a Commander sized 1911.