Kind of a trick but fun question.
Mine is a Remington 12ga 870 shotgun - accounted for over 1000 ducks, pheasants and quail
2nd is a Gamo pellet gun - 256 squirrels in my back yard and counting.
What say you?
Probably two rifles I no longer have:
Cogswell & Harrison .375HH. Wonderful guns and I've had several. One in particular was my backup gun for Africa on about ten safaris. As all of you know, when you bring a 375HH, a large bore, and a small bore, you inevitably shoot 90% of the game with the 375HH out of convenience. It surely has killed around 40 animals in Africa.
The other was a Dakota 7x57 single shot. (I now own its identical twin) That gun has been everywhere with me, bear in Canada, elk in New Mexico, Pronghorn too, deer in Wisconsin, and of course dozens upon dozens of plains game animals in Africa: kudu, hyena, dozens of impalas, warthog, duiker, zebras, and a lot more.
Honorable mention, I got a Marlin .22lr model 60 when I was eight years old. Between 8 and 14 years I'm sure that gun about 20,000 rounds and buckets of miscellaneous creatures.
All a man needs in this world is a 7x57, a 375HH, and a 470NE. Everything else is just a specialized derivative of what those three accomplish.