My father taught me to only own 4 types of ammo. He says you need a plinking/small game round, a Large game round, a shotgun round and a Handgun round
For him it was 22LR, 30-06 12ga and 45acp
For me its 22LR, 45/70, 12ga and 357 magnum.
If I had to pick just one it would be 357 magnum.
Keeping things simple is a good thing you can stock up on less kinds of ammo. Wish I could get my wife to agree but she loves her 380acp