The most underrated cartridge might be the one some ignorant one rifle and single box of shells per decade non-loony living in Montana would choose. Where he lives he's likely to encounter any size animal worth shooting in NA, and probably choose a single .270 Win or .30-06 wood stocked bolt gun and never know he was over- or under-gunned for what he might encounter. He would sleep blissfully at night not worrying about whether or not to punch out a perfectly good 223 Rem, or whether his 105 grain Amax would stabilize in his "marginally-fast-enough-twist" 6mm-06.
So I cast my vote for the most underrated cartridge as the one you would take out if you knew you would see anything from prairie dogs to grizzly bears, but still felt the need to supplement with multiple, overlapping cartidges back home in the gun safe.
