375 H&H.
Everything 26 caliber and up is designed for the same game animals. With the right bullets any of the 6.5's and up are suitable for all but about 4-5 animals on the planet. If you want to hunt any of those 4 or 5 then it is time to move up to 375. And both the 6.5X55 and 7X57 have been used successfully to kill every animal on the planet, even those 4-5 they probably are too small to be used on. Remember WD Bell killed over 1100 elephant in his career. The 7X57 was his preferred rifle, but he also used 6.5X55 and nothing bigger than 303. Late in his life he stated that had the 308 been available that it would have probably been his favorite.
