If I'm not mistaken, the 9mm was developed right before the turn of the century. I could be wrong.
1. .45 ACP
2. .357 magnum
3. .32 ACP (ushered in the modern "pocket pistol")
4. .50 AE (ushered in the big honkin' handgun cartridge above and beyond what's reasonable, i.e., the .44 mag)
5. .40 S&W (really the first design intended primarily for defense/police use AND designed with HP bullets in mind)