30-06. I pretty much only shoot old milsurps these days. Tuned up correctly, there is about nothing they can't do.
1) Components are always available, even during the pandemic+protests craziness. With careful lower-pressure loading, and annealing every 4-5th firing, the brass lasts just about forever. Very, very flexible in powders you can use helps with supply shortages. And when all the other projectiles had been absent on store shelves for over a year, a box of .308 something was always there whenever I stopped by.
2) Want to shoot matches? vintage service rifle gets the blood pumping just as much as f-class or anything else, and really teaches you your own limitations as well as that of your equipment.
3) Hunting? you bet ya. nothing like taking a deer with an m1 garand. for people in less forested areas and longer shots...a well tuned 1903a4 clone is very well balanced and practical.
4) It will be a long time before surplus parts dry up, and there are many good references available to keep these old war horses running.
5) the satisfaction when my 100+yr old, well loved, wood-encased, battle worn, rifle outshoots someones high dollar "precision" rig.
Just plain fun, versatile, and practical.