I vote for the 222 Remington Magnum as the most underrated cartridge. It has more horsepower than the 223, but nobody even mentions it anymore. As for what was said about the 223 and 308 being most popular, that has nothing (or very little) to do with the actual performance of those rounds, but more because they are acceptably sized, give acceptable performance, are small enough to allow for more ammo being carried and are therefore our most recent military cartridges. And that is the basis for why you and I are using them. If it wasn't for Uncle Sam deciding on those two, you might not have ever heard of them. Neither is 'best of the caliber'. They've just been tweaked till they have maximized their potential peformance.
Ya know...looking back, just maybe the 6mm Lee Navy was the better cartridge choice than either the 308 or the 223, except for that rimmed case. Get rid of the rim and get a new bullet design and blow out the shoulder and you have a pretty darn decent round. Likely better than the 223 for most uses, though maybe not as good as the 308 for those really long shots. Just thinking.