I have killed kudus with several calibers : 6.5 Creedmoor, .30-06, .300 Winchester magnum, .30-378 Weatherby magnum, .338 Lapua magnum, .375 Holland & Holland magnum and .458 Lott. All worked great but for a trophy hunt, if I have to choose one caliber, it depends where I will hunt the kudu.
If it is in a mountain area ( like in the Khomas in Namibia), I will take my .300 Winchester magnum with a premium 180 gr bullet. If it is in the thick bush where shots are usually close, I prefer to take my .375 H&H magnum with a good 300 gr bullet.