If I could only have one and it would be required for everything, I'm going with my 375 H&H Mag. With solids you can use it for the very smallest and the very biggest and with soft points you can use it for everything in between. I'm the first to admit that it is too much gun for most of North America, but I also hunt Africa every couple of years. So I need something that can be relied to take down game up to elephant. It stands to reason that something that will kill an elephant will also kill a deer. The reverse cannot be said.
If I can't have my 375 H&H, then I'll take my 9.3x62 Mauser. What can be said of the H&H can also be said for the Mauser.