As has been mentioned, there is no perfect caliber. A single day hunting might need 2-3 "perfect" rifles, depending on the range, terrain and wind. Hunting in Africa would complicate the issue of "perfect", since the quarry can change in 2 minutes time. I've given this question a considerable amount of thought over the last 30 years, and have come up with a "set" of rifles that I can choose from for just about any task at hand. I'm still looking for #4 below, but own the others and all shoot 1/2 MOA or better.
In order of smallest game/lightest skin/closest to largest/heaviest/furthest
1)243 Winchester (55gr and 90gr bullets)
2) 7mm Rem Magnum (160gr Accubond bullets)
3) .338 Edge (300gr SMK bullets)
4) .375 H&H / .375 Ruger / .375 RUM
5) .338 Allen Magnum (probably re-barrel to .375 allen mag in 5-6 years when barrel is toast).
If I was visiting Africa, I'd bring #2 and #4.
I bring #1, #2, or #5 when going after deer, the terrain and situation will determine which one I use.
I bring #2,#3,#5 for Elk (it would be #2,#4,#5 if I had #4)
For coyotes, I take #1,#2 (and a .22lr
)
So answering your question "which caliber should I buy?" is pretty much impossible. For expensive game up to 400 yards (including Africa), I'd just get #4 if you can handle the recoil, add something similar to #2 for the plains game.
Scopes are an entirely different discussion. I really like the 4.5x14x40 Leupold for general use. I wouldn't use that much magnification for dangerous game.
I use the Nightforce scopes on my sit and wait guns (ie. Long range style,NP-R1 reticles)
The 1x4 has a HEAVY duplex, on 1x I can keep both eyes open and focus on anything from the end of the barrel out to 50 yards easily. At 4x, it's not an issue shooting to 400yards.
The others have standard duplex (#1,#2)
I have the following scopes for the above rifles:
1) 2.5x8x36 Leupold
2) 4.5x14x40 Leupold
3) 3.5x15x50 Nightforce
4) 1x4 Leupold (I have the scope, just looking for the right rifle) (will upgrade to 1x4x24 Nightforce at some point).
5) 3.5x15x50 Nightforce
Hope this is of some help,
AJ