I am starting this as 'what is the most 'under-rated' cartridge?
I'll start with the lowly .30 carbine caliber. it is the most fun per caliber, medium power, easy to reload, and it comes in lightweight gun.
I think this is the most underrated caliber because it comes in the Carbine rifle, not known for accuracy.
Had this caliber ever been offered in something like H&R single shot, a light bolt gun (mini-mauser), or a pump, this would be a absolute hoot to shoot.
the cartridge is more capable than the common gun it comes in.