My opinion (we all have one) based on very limited experience:

Plains game only and excluding giraffe,

A .30-06 or any of the .300 mags will do if you are willing to pass up unfavorable shots on larger species. Rear quartering away shots on eland, zebra, wildebeest, or kudu come to mind. My hunting opportunities will always to too few and too short to have to pass up such shots so I leave the .30 cal's home.

A .375 (or a 9.3x64) is just more than I can justify using on these animals.

Terrain matters.

South Africa North of Jo'berg (bushveldt). Ranges are almost always less than 200 yards.

Standard cartridges from .338 to .366 (9.3) like .338-'06, .35 Whelen, or 9.3x62

Southwestern part of South Africa (more open country)

Faster cartridges from 8mm(.323) to .358 like 8mm Rem Mag, 8x68, .338WM, .340 Weatherby, or .358 Norma Mag.

Obviously, any of the faster numbers could be used in the bushveldt also. Otherwise, arguing among these fine medium bore and any others I forgot to mention by name is probably picking nits.

Always pick your rifle, cartridge, and bullet for the biggest, toughest species you are hunting. Then just live with the fact that the 150lb impala you just shot with a heavy premium 9.3mm bullet is going to run farther than he would have if hit by a nosler ballistic tip or some other simple cup-and-core bullet out of your .257 Roberts.