Easiest: A SpongeBob game I played when I was about 10. I clocked the game in around 2 hours, and it was a simple point-and-click game.
Hardest: I'll give an honourable mention to Call of Duty 4: Modern Warfare, simply because of the ***** that was The Epilogue on Veteran. Scratch that, all of it on Veteran. Though, I think that Ghost Recon: Advanced Warfare gets the cake, simply because on Hard, the enemies always aim for the head, and one shot kills.
Though, I'm guessing Brothers in Arms: Hell's Highway will be a pain, since on Realistic, they have no checkpoints during a mission, and you die in a shot. Though, I haven't played it, but this is just what I've gathered from it's predecessor.