Apparently there a quite a few "best" cartridges in my opinion, as I own quite a few!
I do hunt a wide variety of game, in a wide variety of environments...so I have rifles (levers, single shots and bolt actions) that will, and have, worked very well on a lot of game over the years.
From bunnies to bison, from the thick bush where 50 yards is a long shot, to the open prairies and mountain alpine where you can shoot game as far as you are comfortable with.
On big game, for me, it has been 15 yards out to 475 yards, with 20 cartridges ranging from the 250 Savage to the 376 Steyr. I have 9 more rifle cartridges that I still need to take game with!

My first 7 big game animals taken were with the 30-30, 308 Win, 30-06 and 300 Win Mag. Over the past 20 years, I have only hunted with a 30 cal in 300 WSM, which I prefer to all 30's, except the ol' 30-30.
Glad to have the option and ability to choose what is best for us for any game, in any environment!
Favourites include the 250 Savage, 6.5x55, the 280 Rem, 7MM STW, 338 Federal, 356/358 Win and 375 Win. (Interestingly, I am down to 4 magnums)
The cartridge I have more trigger time with, other than a 22LR, is the 6.5 Swede.
And the rifle that has taken the most game to date has been the 7MM STW.