Reduce Flickering atm, haven't tested Sharpen yet. I wonder however how big the differences are between each options. I played the first hours on Normal, later I changed it to Reduce Flickering. Can't really say I notice anything.
I left mine at normal last night when I was adjusting everyting, I may try the Sharpen and Reduce flickering to see which appears better. My bet would be the Sharpen setting.
Reduce flicker turns off the temporal Anti Aliasing
Normal has it on. You can see the screen flicker when the game is paused. It smooths the image a bit by alternating pixel offsets
Sharpen, not sure