That actually is a difficult one to answer for me!
When I was a much younger bloke the old .243 Win was it, as time progressed I added rifles of different calibres so it was like picking the right 'tool' for the job, or matching the calibre to the game. Never tried full loads on anything 9.3mm & up yet for obvious reasons on anything big enough in my sights yet.
Reduced loads in these calibres are great fun on porkers & deer
Especially shooting a .500 3" NFB 440gn Woodie load at pigs, they really go SPLAT!
Very hard to go past my old P14 in .303 British, completely dependable & never fails to bring home the bacon......pun intended!