I think the most overlooked underrated rifle cartridge today is the 30-30.
My very first thought. Wasn't underrated in it's day but today people would have you believe it's a bunny gun.
And how anyone can say that the 2 most popular cartridges for their intended roles are the most under-rated baffles me. But the OP seems stuck on long range so sticking with that the most underrated long range cartridge has to be the .22lr. People seem to think it's a 75, maybe 100 yard round but you want to have some fun stretch it to 200-300 (even 400-500 if you want)yards. Cheaper, more ranges available and it takes just as much talent to hit what you want at those ranges as using centerfires at 1000.