Don't actually own any rifles so-chambered myself, but .223 is the industry standard these days.
Probly was .222 or .22-250 when I was a kid...

My Sako L46 .222 has been put to effective use in several macropod culling operations, mainly Agile Wallaby, and I have helped a mate shoot Eastern Greys professionally for human consumption on several occasions in Central Queensland using his Winchester Model 70 .223.
Accuracy for 100% head-shots out to ~200m max (but a lot less in practice) is the primary consideration.