Yeah, gotta go with .25 acp.
With honorable mentions to:
--.45 GAP (obvious)
--.32 acp (now that new .380s are smaller than the old .32s, why?)
--.460 S&W (it's a .45-70 with inferior pistol bullets)
--5.7x28 (ehh, ok, I see what it's for, but doesn't help those of us without full-auto)
--.357 Sig (good idea during the 10-round limit era of 94-04, but not anymore)
--.300 RUM or any .300 "ultra-mag" (overbore, overly brutal, and under-performing)
--The new Ruger short mags (late to the party; already been done to death)
--.450 Bushmaster (what's wrong with the .458 soccom; I forget)
even better. .380 auto, why have a gun that no one can find bullets for?
Ouch!
I'm reserving judgment on the .327 Fed mag (gets you 1 more round), the .204 Ruger (perfect goldilocks harmony between velocity & BC, blast & recoil, for varmints?), WSSMs and RSAUMs (fit in an AR rifle), WSMs (no belt), .30 RAR, and some others....