9mm. Here's why:
1. 9, 45, and 40 all have the same effectiveness against bad guys. Shooters don't want to believe this, but MEs and ER docs can't tell the difference. Neither can people who get shot. "Stopping power" etc. is myth.
2. 9 carries more ammo than 45 and 40. In a poop-storm, more bullets are life-savers.
3. 9 recoils less, allowing faster more accurate shooting. Faster more accurate shooting means you live instead of die.
4. Ammo is much cheaper so you practice more.
Net result: If you choose .45 or .40, you get nothing in return for important things you give up.
So if you lived in a state that restricted magazine capacity to 10 rounds, you would still choose 9mm over .45 or .40?