I have different favorites for different types.
1. For hand guns, 9mm.
The small size, and low recoil is (for me) the best all around combination.
2. For shotgun, 12 gage.
The entire reason for a scatter gun is to go big. You can still tailor your load with shot size, shell size, and going with slugs or not.
3. For bolt guns, 7mm Mauser.
A classic that lost favor, it can still do everything needed, and with good accuracy, and less recoil than many more modern calibers.
4. For defensive rifles, 5.56mm... and in a a DI AR.
Hi velocity, high magazine capacity, low recoil, good accuracy, enough range for most combat engagement distances without carrying too much weight...
For 90% of what you need, it just works.
5. For lever guns, .44 mag.
Lever guns are (to me) meant to be light, fast handling carbines for close to medium ranges. A .44 mag round is scooting pretty well out of the 17" barrel on my Rossi 92. It's good for 100, maybe 150 yds on deer sized game.
With 10+1 capacity it also doubles as an anti-bad guy gun, something it often did throughout it's long history. Good enough for me.
6. For pests, or long term survival, the world famous .22lr.
The last round of firearm ammunition ever fired by a human will one day probably be a .22lr... May not even be on this planet... Need I say more?