What difficulty makes specific games the most fun?

I play pretty much all my games more than once, and I find that different games have different difficulties that make the game seem better.

I find with games like CoD, or Battlefield that playing on the hardest setting makes the game more realistic and therefore more fun. It makes it seem like you really are hiding out in trenches, moving up when you can, and you die in just a few hits. Obviously not perfectly realistic, but the best you can get.

But with games like Halo, Vanquish or any hack-and-slash I find the easiest difficulty makes the game more fun. I think this is because the story makes you out to be some huge badass that takes on entire armies single-handedly and walks away without a scratch. I love being able to just breeze through the game like I am that badass.

So what about you? What games, that you've played, have certain difficulties that make the game better?