Tip, any game will give you 3 times the gs of any achievement with little effort if you also play it in the other region's consoles. Thats how Stallion did it.
I usually play single player of all the games I buy. Ass load of achievements. Especially if you play on the hardest difficulty. I am not a hunter or anything but somehow I have like 25k gamerscore.
Borderlands
King Kong
That one Avatar last airbender game (I think it's 1k)
All CoDs
Battlefield
TMNT
Terminator Salvation
Lost via Domus
Basically all sports games
Avatar: The Last Airbender: The Burning Earth
NBA 2K6
NHL 2K6
MLB 2K6
King Kong
That right there is a nice 5,000G boost right there. Probably take like 2 hours for everything to be done. King Kong will take a few hours though on it's own. Unless you skip to just the chapters that get you the achievements (you use a cheat). But you'd only then get 800G instead of 1000G.