Stallion83 has the highest gamerscore I've seen, with 212300 Gs. His is legit too, you can check if someone has gamesaved by seeing if large amounts of their achievements have no 'date gained' on them, as far as I know his are all legit.
The Mygamercard leaderboard doesn't actually remove gamesavers though. There is a gamesaver-free leaderboard
here (link).
As for me, I don't like how artificial and inconsistent the whole achievement system is. I actually like metagames, but achievements are not for me.