.22lr by a mile. I doubt I shoot more than 500 rounds of any other caliber in a year and it's not unusual to shoot 500 rounds of .22lr during one visit to the range.
.22LR
...Duh
As a reloader, I reload for and enjoy .45 acp, .45 Colt and more recently, .41 Mag. I have dies and components for (but have not yet started reloading for) .380, 9mm and 38/357.
That said, I still shoot at least five times as much .22LR as any other individual cartridge.
considering that I currently reload for 26 different rifle and pistol cartridges (and have a few more I don't currently load for) that ought to tell you something.