.22LR - because I have 40K + on hand and there's lots of good work you can get done with it...
.300 Blackout - economical, shoots cast great in the single shot and jacketed from 110-240gr great with small powder charges; I'm loading cast right now for far less than good rimfire and it shoots under MOA with the SS and the AR both; I can make cases from some of the .223 that I have half a dozen 5 gal buckets of sitting around; (this could easily be a .32-20 if I didn't want to mess with the AR)...
.308 Win - both prolific and available, lots of effort has gone into developing fantastic ammo for this caliber over the years; handles most big game chores efficiently and if I want to shoot precision rifle this sure gets the job done; (this could be anything from .243/260/7-08/3006/270/280/7Mag - whatever blows up your skirt)...
9.3x62 - this has become my Thumper for a bunch of reasons, I also sort of group the .45-70 in here mostly for the cast bullet capability though range is reduced here for the most part (I know, take it easy BPCR folks... LOL), but this could be any of the medium bores from .338 to .375...
10mm Auto - if I can only have one handgun caliber, this is it. I can shoot .40 out of it and if I'm shooting Glock I can get a .22 conversion unit for it - winner!!
20 gauge (I know it's supposed to be 5 but 20 is a gauge not a caliber... LOL) - I don't do waterfowl, anything I need a shotgun for gets handled by the 20, including shotgun only deer...