I have found the vast majority of PHs with whom I have hunted to be quite professional, though admittedly that has only included two hunts in South Africa. Because I am a US citizen, the option of hunting "alone" doesn't really exist in any practical way in Southern Africa or Europe.
PH training, at least in my experience, is pretty darn thorough in Namibia, Zim, and Zambia. My PHs in Moz were all originally licensed in Zimbabwe.
I have no problem being called a "client." I certainly prefer it to term "sports" which is the name we used for clients when I guided waterfowl hunts back in the day. Lord knows the term "hunter" certainly didn't apply to all of them.
With respect to the OP's question, Nick Nolte, with whom I have hunted on three occasions, carries a .470 Krieghoff when guiding for dangerous game. Jamy Traut, with whom I hunted in the Caprivi, carries a .500 by the same maker. My son's PH in the Zambezi Delta also carried a Krieghoff .500, though my friend Boet remains true to his Mauser 66 in .458. Of the remainder, CZ's in either .375, .416, and .458 WM or Lott predominated.