2015, I recommend using a multi-colored crosshair ala Global Warfare (mod for Half-Life).
Only one part of the crosshair can be seen depending on where you're aiming..
For example : your crosshair (in, let's say, Counter-Strike) is white. When you aim in a direction where there is alot of white / pale colors, you tend to have trouble seeing the crosshair. I recommend it be made of several colors (vertical and horizontal line made one color, diagonal lines another) to get rid of the problem.
Of course, aiming with the weapons' sights would be fun
