After many years searching for this elusive grail, my personal answer is .257 Wby, .300 Wby and .375 H&H.
- The .257 Wby 100 gr TTSX is absolutely perfect for all small & medium PG and likely the best for mountain hunting (Vaal Rhebok, Mountain Reedbuck, Mountain Nyala, etc. as well as Himalayan Tahr, European Chamois, American Mountain Goat, etc.) and it shoots laser flat out to 400 yards with barely any recoil to notice (equivalent .270 Win) which makes it extremely easy to shoot well.
- The .300 Wby is by a large consensus the best .300 there is when combining reach / power / ammo availability. It does everything with a 165 gr or 180 gr TTSX and hits a tremendous blow with a 200 gr AFrame. You can replace the .300 Wby with a .300 Win if you prefer; same bolt head. Either are needlessly powerful for most PG species, but shine on the larger ones (Kudu, Wildebeest, etc. as well as European or New Zealand Red Deer, American Elk, etc.).
- The .375 H&H satisfies all minimum legal requirements for DG and has done it all for over 100 years. It is not ideal for sure climbing mountains after Vaal Rhebok, or stopping a charging Buffalo, but it can do it, and it is perfect on Lion, undisturbed Buffalo, Eland, and good on Leopard too with a quick-opening bullet like the 250 gr TTSX.
You can replace the .375 H&H with the .416 Rem if you want more punch, it has the same bolt head and cartridge length, however a lot of people are disagreeably surprised by the recoil of the .416. Many folks show up in Africa with a .416 they are afraid of and cannot shoot well, and would be much better served with a .375 H&H...
You can also replace the .375 H&H with the .458 Lott if Elephant is on the menu, it too has the same bolt head and cartridge length, but the recoil enters another dimension with full loads. To many, it is simply too much...
I have played over the last 40 years with many PG calibers, and I still currently own .24, .25, .26, .27, .28, .30, .33 PG rifles, but everything considered I have come to the conclusions that a progression of 0.05" makes sense when going to the next caliber: .25 to .30 to .35 (or .375) to .40 (or .416) to .45 (or .470) to .500. Certainly there is a strong logic for .25/06, .30/06, .35/06 (i.e. 35 Whelen) but I personally prefer a bit more reach and flatter trajectories, and the .35 fails legal caliber requirements.
My own Blaser R8 battery includes .257 Wby, .300 Wby, .375 H&H and .223 Rem barrels. I hesitated long and hard between .22 LR and .223 Rem for practice and decided to go .223. Ammo is not very much more expensive than quality .22 LR, and still dirt-cheap compared to the Wby calibers, and the .223 extends the practice range to 300 yards, which the .22 LR simply cannot do.
I have no need for .416 Rem or .458 Lott barrels - which Blaser also offers - because my Pelican 1700 hosts both a R8 with 2 barrels AND a Krieghoff double .470, in a case that weighs less than the 50lbs., and measures less than the cumulated 62" linear total, maximum airlines no-fee luggage allowance.
This flexibility is why I finally walked away from double square bridge magnum Mauser system rifles. With them I could only bring 2 calibers to Africa. Now I bring 3. Vaal Rhebok is always on the list, so the .257 Wby barrel comes; Buffalo is always on the list, and I like to hunt them up close & personal, so the .470 double comes; and depending on what else I hunt, either the .300 Wby or .375 H&H barrel comes.