If I were to only have one and already had a .30-06 on the rack, it would be a .375H&H, or a coin flip between .338-300wm and .338RUM. I love fast and flat trajectories, but when you're talking out of state trips and the many thousands of dollars that entails, a moderate speed bullet which will anchor your game quickly is the way to go. Especially if you plan on multi-species hunts where any time spent tracking game is less time you have available to hunt the next species.

If you do plan on Africa in your future, a .338wm or .300wm will make a fine PG rifle, but consider a one gun safari where you only need a .375H&H for everything.

Another thought, since I went through this myself. I spent most of my 20's and early 30's hunting with the "ideal rifle" for each game species, but then I found myself feeling empty whenever I held one or the other rifle. Instead of stacking a lot of memories up with "my rifle," everything was diluted out among a bunch of different rifles, some of which hadn't even left the safe in a decade (Win 70 .416rem mag which took a water buff in Argentina when I was 24, hadn't been hunted since then, for example). I never really want to sell any of these rifles, since I have that ONE major memory attached to them, so my safe gets more and more full and my wallet stays empty buying new rifles "ideal" for each new hunt. So now I've changed gears. I've set to build up as many memories as I can with one or two rifles. I ordered a handful of barrels for my wife and I, and two Ruger controlled round feed M77 Hawkeye's. They'll be capable of 7mm, 338wm, and .416 Ruger for hers, and .300wm and .458wm (maybe Lott, still working on that) in mine.

So you might consider getting a .300wm or .338wm for now, then plan to change it to .458wm when you go to Africa, and take that .30-06 over as your plains game rifle. Getting a .300 RUM now would let you convert to .375h&h or .458 Lott in the future. That'd give you the chance to use one rifle for your Alaskan hunts as well as your African hunts in the future, without dropping another $1,500 or more on a different rifle later on.

ETA: On the other hand, if you pick up a .416 Rem, Ruger, or Rigby, or .375 H&H now, already owning that .30-06, you'll have a devastating Moose and Grizz hammer, and also won't have to change a thing when you finally make it to Africa.