So........... let's include the bullet perspective............
The .30-06 is wonderful and made its name pre and post WW II in Africa, shooting 180 gr or even 220 gr bullets that typically fragmented upon impact and generally lost anywhere from 50% to xx% of their weight during penetration. Even using the Nosler Partition as a modern (post 1945) reference, the loss of the front core and about 40% weight was standard.
Modern bullets such as A Frame or TSX / TTSX (or equivalent) that retain 95+% of their weight penetrate a lot deeper. So an "old" 180 gr load is the equivalent to a modern 110 gr load in term of penetration (
I proved that one to myself with the .25 100 gr TTSX on a dozen PG species up to and including Roan), and an "old" 220 gr load is the equivalent to a modern 130 gr load.
What this means is that modern .27-06 loads do the same as the old .30-06 loads in terms of penetration. Calling the .27-06 by its commercial name, the .270 Win, with a 130 gr TTSX it will kill any PG on earth, and it will provide an additional safety margin with a 150 gr when going after heavy PG. Conversely, it can go down to 110 gr for MG (mountain game). I am thinking Vaal Rhebok here...
The .270 130 / 150 gr will also shoot significantly flatter than the .30-06 180 gr / 220 gr, still without paying the price in noise and recoil of a magnum.
If the .270 appeals to you Carter123, i will second Cam Moon, this is one of the very best PG calibers there is, between 110 and 150 gr.
If a metric caliber appeals to you Carter123, I will second IvW and say that the 7x64 too is about perfect, between 120 and 160 gr.
In reality, the .270 Win (bullet diameter .277" or 7.03 mm / case length 64.5 mm) and 7x64 (bullet diameter .284" or 7.21 mm / case length 64 mm) are virtual twins and both will match and exceed the performance of grand old 220 gr and 180 gr .30-06 loads.
PS1: there is no flies on the grand old 7x57, of course, but the 7x64 shoots significantly flatter, which is not a bad thing.
PS2: I too believe that the .243, while a great cartridge is a little light to be a true "one and only" rifle for PG. Wildebeest, Hartebeest, etc. can get big...