I think, perhaps, the "most coveted caliber" is the one that a hunter doesn't currently have in his gun case.
Having said that, after reading many books and stories about hunting in Africa (Hemingway, Ruark, etc.), my first African big game caliber rifle was a Model 70 Safari Express in 375 H&H, which is one of the historic safari calibers. I have always wanted a 416 Rigby, so Santa brought me one this year. I would say that those two calibers are probably the most talked about, written about and historic African big game calibers. Why? Because, when the British colonists began hunting big game in Kenya and Tanganyika (Tanzania) in the late 19th and early 20th centuries the London fine gun makers, especially Holland & Holland, John Rigby & Co., Westley Richards, Purdey, etc. began making rifles especially for African hunting and the 375 H&H and 416 Rigby were born of that era and carried by the famous hunters of that period. The same could be said, but maybe to a slightly lesser degree, regarding 404 Jeffrey, and the nitro express calibers.