I use a .500 Jeffrey open sighted rifle custom built on a left handed Weatherby action by Sabi Rifles in Nelspruit RSA. I'm perfectly happy with it and don't want anything else. I guess I get to see a lot nice rifles owned by clients, but I'm more than happy with my the one I have. The other PHs who work with us use the following:

.458 Lott
.458 Stewart (3 inch)
.416 Rigby
.416 Rigby

I wouldn't allow any PH who works for us to use anything smaller than a .40 calibre when conducting a hunt in any DG area.....but I am very choosy indeed about every aspect & standard of our PH's.