The .221 Rem, as named before. I think its a better .22 Hornet than the Hornet, and probably what most people are really after when they get a Hornet.
I've wanted a Kimber 84 in that round since, well, 1984. Around 1989, I had a Remington 600 rebarreled to .221, but its only a so-so job (for starters, one rifling groove is clearly deeper to the naked eye). It still shoots OK, so the potential is there for a very accurate cartridge/rifle combo.
I'd like to also nominate the .44 AMP (Auto Mag Pistol) cartridge. I always thought a rimless .44 Mag would be useful in box magazines for carbines. Something like the Ruger 44 semiautos but with the capability of using a bigger magazine.
While I'm at it, give me the .357 AMP also. That's the .44 necked down, and arguably the more interesting cartridge. It is almost the same as the .357-44 B&D, also named earlier. I always thought it could be a neat carbine round.
Having got a Single Six in .32 H&R Mag recently, I see the point of those asking for the .25 Stevens and similar earlier in this thread. I have had some thoughts of using the .32 Mag as sort of a reloadable .22 LR, and that's what a .25 like that would be.