There are some 80-90 million SWF files from the last eleven years and in ten versions scattered across the web. Versions 7 to 9 are all still widely used on popular websites. Version 6 is quite common, and even versions 4 and 5 are still found - largely for classic games or applications, but sometimes in new files generated by alternative SWF compilers. (The openoffice.org SWF export still uses version 4, though it produces poorly formed SWFs). Various web archives keep version 1 and 3 files, from Flash's pre-actionscript phase, though you very rarely see these in active use.
But while it's difficult to say which is the most widespread version out there, it's easy to say which is the least common. SWF version 2 files are fantastically hard to find. I have found exactly one file, and it's an advert from Macromedia itself. Moreover, this particular example uses a java applet(!) to interpret and display the original SWF file, so that the Flash browser plugin never comes into play at all. Fortunately, there is a way of getting the original file...
This SWF2 rarity is a shame, because several interesting tags were introduced for version 2, only to be dropped immediately for version 3, including the DefineButtonCxForm tag and the original PlaceObject tag. Testing Gnash's implementation is correspondingly difficult. Luckily the Macromedia demo file uses both PlaceObject (unsurprisingly) and DefineButtonCxForm, so Gnash can happily change the button colours as expected.