Based on what I own -

Light - .308 - it's not perfect, but plenty of good ammunition available for it, reasonably priced, and it doesn't beat you up. .270 or 7mm Rem mag if I couldn't own military chamberings
Med - .338 WM. Like above, it might not be perfect, but it does the job against anything in my part of the world.
Heavy - .458 Lott