If I were you I would go check out a site called True Achievements. It is based on calculating what a gamerscore of a game should be on account of how many people have scored achievments to haven't.
For example a game like Bejeweled 2 has a Gamerscore of 200, but it has a True Achievement score of 1,107 because so many people have it and only have about 3 achievements.
So go visit and go to the games section (Which I have added bellow) and arrange by TA Ratio and look for anything closer to 1
http://www.trueachievements.com/games.aspx
But remember to check how many people have it, sometimes a game will see to have a low score due to only 30 or so people having it and all of them having it at 100%
Good luck ^_^
Oh, and I might add that most games you find with an easy Gamerscore are usually of the bad game persuaision. This is due to a fact that some time ago developers realised that easy achievements mean an extra 20% in sales (I am not joking this is a real figure) So if your making a bad game and need easy sales make easy achievements.
One easy one I got was TMNT, it wasn't to brain depowering plus only about 3-4 hours of gameplay, just plug in a USB play your own music over the top and it isn't too bad.