Now this is a nice little idea. After registering and inputting your Xbox 360 Gamertag on True Achievements you see what your real Gamerscore is, based on how rare your unlocked achievements are and how hard they actually are to obtain. Have a read of the about page – it’s quite funny.
Turns out my best achievements are for finishing Lost and Ninja Gaiden II. Lost wasn’t exactly an impossible mission so I’m guessing that not many people bothered to finish it.