For a one rifle safari it depends a little on what and where I would be hunting. You cannot go wrong with a 375 H & H. If your firearms do not show up your borrowed rifle will probably be a 375, or an 06 if only after plains game. Decades ago, my first Africa trip I took three rifles, a 300 H & H, a 375, and a 416 Rigby to Zimbabwe. I only used the 375 for everything. That was the last time I took three rifles, for me two are plenty. I had hunted all over Africa many times before focusing on South Africa for indigenous plains game like the Tiny Ten, Blesbok, Bontebok and so forth.Took only my 300 Win Mag. Taking 42 animals in a month. An 06 would have been fine. someone canceled a Timbavati buffalo hunt at the last minute so I borrowed a 375 an took a 46 inch buff with one borrowed round.
As someone mentioned, borrowed ammo may well be a catch all accumulated over years. I was headed for one of my trips to the C.A.R. and on airport arrival the Air France clerk noticed one of the trips legs did not show I was traveling with firearms. She stated I could not take any of my rifles even though the other seven round trip legs showed them, I indicated obviously they had just missed the one leg. She drew herself up and stated Air France did not make mistakes! I borrowed a 375 on arrival. it was beat up and the collection of ammo gave me misgivings, I had one of the most successful hunts ever! One round exploded on a Western buffalos shoulder and it charged. My second round dropped it instantly. Turned out the buffalo had been previously wounded by poachers. We found a truck battery terminal lodged in its neck. I still have that homemade bullet.
the 375 H & H is a great cartridge But I usually take my 300 Win Mag and my 416 Rigby. Kindest Regards