Guns are tools and a good mechanic has a big toolbox full of tools. An excellent mechanic has many tools plus specialty tools for specific jobs.
Most of us fall into the good mechanic area. We have guns, more than one because it takes different calibers for different situations.
Most everyone has a Crescent wrench in their toolbox (Crescent is really an adjustable open end wrench, not Crescent, a brand name BTW).
My Crescent wrench is my 223. I use it for all sorts of stuff, from target practice to 100 yard match to varmit removal to even deer hunting (it's perfectly capable with the right projectile and handload), plus it's cheap to shoot on factory ammunition and real cheap with handloads.
Guys like Shawn Carlock and Jeff and others, have specialty guns (tools) for specific purposes. You don't go plinking with a Chey-Tac or a 338 because you can't afford it and those are guns with a specific purpose, much like an H&K with supression and night vision is, or those fancy and high priced, specific use Snap-On tools that I'd like to have, bit really have no use for.....
That brings us to the answer which is 'whatever caliber you, as the shooter are most comfortable with and can afford to shoot frequently'.
Something to ponder.................