If you go about it from the other direction and search the GT threads, I think you'll find that the Glock 19 (9mm) is THE most popular Glock. It also may be the most popular single model handgun in America (there may be more 1911's but they are spread over many brands).
Not trying to get into a 45acp vs 9mm war, but with the current self defense loads the 9mm appears adequate.
And as many others have noted, it is the cheapest ammo which will allow you to shoot more. Also, as a NATO mandated round, whenever there are NOT wars going on, we have a super surplus of it and it gets cheaper.
It is also the cheapest to reload if you get into that later. At $5-6 per box of 50 of reloaded 9mm you can really shoot a lot.