My ideal battery would be based around a take-down 9.3x62 (or a .375 H&H).
For a dangerous game safari I would take the first rifle as a back-up and add either a .404 (or .458 Lott) magazine rifle or a .450/.400 (or .450 NE) double rifle.
For a specific open country plains game hunt I would take a .300 Win Mag.
I think that takes good account of usefulness, effectiveness and potential back-up for ammo availability.