Bright green. It's the color I've always used in games that let you pick it. I think CS poisoned me with the need to have a green crosshair. (and having my main weapon on the third key not the first! god so many deaths have been caused by me forgetting that or not re mapping my keys.)
mine is white, i just looks horrible if there is something ugly and pinky in the middle of the screen. never had any problems with aiming or something, even in snow
i use red its stands out well against most backgrounds :) however if i could turn it off i would, for realisms sake!