GLOCK 23 PEOPLE!!!!
WHY: You can put a 9mm barrel in it and shoot 9mm ammo! Let me say that again: YOU CAN DROP A GLOCK 19 BARREL IN IT, AND SHOOT 9MM AMMO OUT OF IT. You don't even need a conversion barrel! Personally, I have a conversion barrel because they are better barrels but my friend has a Gen 4 G19 and I took it to the range and pulled the G19 barrel out and put it in my Gen 3 G23 and it shot GREAT.
GEN 3 G13 is also the cheapest Glock available. BOOOM!!
GEN 3 G23 can shoot TWO calibers with same mag: 40S&W and .357 Sig, both use same mag (although Glock labels one the .40 and the other .357, they are the same mag). AND, if you have to use a .40 mag, it WILL WORK with 9mm ammo. It's not recommended, but the gun will shoot.
So, in essence, the G23 is the most flexible gun in the Glock platform and if you buy the Gen 3, it's the cheapest! You can buy conversion barrels for 9mm and .357 Sig and you can also just do stuff like, oh, drop a G22 barrel in it, or a G17 barrel in it and shoot it.
Anyhow, my 2 cents.