While I dislike the premise of the original question, I'll choose from my current collection for the sake of the discussion. To me, which cartridge I like best also depends on the firearm they come in. There is a lot of overlap among cartridges and the jobs they do. Mine are all very Fuddy choices. So...
Brno #5 .22 LR - plinking, training, small game & varmints
Brno ZKK 601 .222 Rem. - as above, adding larger fur bearers like coyote, beaver, and oddly, it's a cartridge recently made legal for "big game" in Sask. but unlikely to be used by me for that purpose.
Sako L579 Deluxe .308 - Deer, Pronghorn, Elk, Moose, Bear. Anything most people call "big game" here in Saskatchewan. I'd really miss my 9.3's, 7mm's and 25-06. But could enjoy successful hunting without their slightly greater power or flatter trajectory.
Browning Citori White Lightning 16 ga. 28" - anything properly hunted while accompanied by my Drahthaar - Upland birds, bunnies, jump shooting ducks ( difficult to not choose the 20 ga. White Lighting Feather instead, but I shoot the 16 better!)
Browning Citori White Lightning 12 ga. 28" - anything that requires steel shot, or geese, 3" shells, or target shooting.
Then I'd have to give up precision shooting with my Sako M85 HB .223, and muzzle loader deer hunting with my handy little T-C Cherokee .45 cal, and the Anschutz .22 mag that is perfect for shooting nuisance beavers, and my .375 Sako M85 bolt action and .400 Jeffrey Merkel 140 AE double rifles that are made for hunting very big critters in exotic hunting locations, and a good Beretta SV10 sporting clays 12 ga. 32" O/U, and a Rossi .44 mag stainless steel M92 carbine that is excellent to keep at our bush camp for bear repellent, a Ruger M77 "canoe paddle" stainless .30-06 that is built tough and perfect for fly-in or canoe hunts in wet, rough locations, and bunch of other stuff that is "just right" for intended purpose. Or fun to use because of artistic design, slightly better performance, or ultimate versatility like my Krieghoff drilling in 16ga/.22 mag/7x57R. And a couple of take down guns that are wonderful for travel by car or air.
Sigh. Can't do it. Not voluntarily anyway.