Well it has to be .303 mainly because I have about 170 rifles in that calibre, give or take a few. (haven't counted for a few years.) and three machine guns.
Next, 577-450, 577 snider, 11mm mouser, 11mm gras, 45-70, 32-40 7.5 Swiss and 6.5 Swede, but only two rifles in 30-06