There's tutorials all over the place to find out how to mod gamerscore.
As for who has the most legit, I could not tell you. I have a friend who has 140,000 GS legit. He plays all day, every day, and had 50+ 360 games last time I checked.
Stallion83 is legit. He's been first for a very long time. Every now and then, you will find a handful of profiles who overtake him, but within one week, you see their Gamerscore go back to 0.
I am under 60,000G and I can tell you that I am not legit. I have hacked at least 3000G.