The hardest game I have EVER played was Geometry Wars.
The easiest game I have ever played was some stupid Lord of the Rings RPG that was totally lame. I beat it in 30 minutes on the hardest difficulty. I haven't played a LotR game since, it was so lame.
Easiest: probably Contract J.A.C.K - great fun though especially if you like the No One Lives Forever games.
Hardest: Project I.G.I. - I gave up in frustration (I hate games where you can't save).