I'd also have to go with CCI .22 LR as the most game killed. Lord knows how many rabbits and squirrels. Though I seldom hunt small game anymore. Just enjoy the walk. For medium critters: just took whitetail deer #50. For me the most successful would be a shotgun slug (latest were the Hornady SST sabot .50 cal FTX bullets in a Hastings rifled barrel). This region only opened up to rifle hunting in 2018. Second for me would be a patched round ball and blackpowder (.490, .530 & .650"). For 15 years that was the only firearm I hunted with, regular & muzzleloading season. Still use them in M/L season after regular. Third would be a two-edged broadhead.

I really don't know how many varmints I have killed with a Sierra #1410 .223" bullet out of a .223 Rem. Been my farm varmint rifle for 40 years and it has ended a bunch.