The easiest achievements for me would probably be Prey. You're using god mode throughout the game so that's well over 600 GS in your pocket already. A lot of the Dead Rising achievements are also easy, but the rest not so easy GRAW is moderately easy. Burnout is moderately hard and COD2 is hard. Kameo is easy initially, but scoring the A grades is hard. Oblivion is another easy yet time consuming game

COD2 is the best hard game
Dead Rising is the best moderate game
Oblivion is the best easy game

For Xbox Live Arcade, Geometry Wars is hard becaue I'm not Marauderz UNO just takes time to get - easy. Crystal Quest is the easiest for me, but may be harder for others. Outpost Kaloki is slightly hard but manageable. I suck at Hexic so that's hard for me

Geomerty Wars is the best hard game
Outpost Kaloki is the best moderate game
UNO is the best easy game

You don't have to shoot them. You can just stay still and wait