There is a fellow here in my town who was a Glock rep for several years. He told me that the G19 is the company's best-seller. The company makes and sells more G19's than any other models. After that it's the Glock 17 and then the 40 caliber models. The 45 models are strong sellers in the states, but not so much anywhere else. However as of a couple of years ago the Greek anti-terrorist unit went with the Glock 21 as it's sidearm. I though that was interesting.

Now this information is from a few years ago and Glenn now works for Colt so I can't say how current it is. But my guess is it probably hasn't changed all that much.