For any shotgun I use the + cross hair scaled to 75 and bright purple. This makes it so if the entire cross hair is on the enemy you will get a meat shot. It also shows spread on no random spread servers(comp servers). This only works with 90 fov. The bright purple makes it so the cross hair stands out.
Almost every other weapon I use a small purple dot, except melees I don't have cross hairs for melees.
My spy cross hairs change colour depending on what class I'm disguised as. Green for offensive classes, red for defensive, and purple for support. I use three different shades of the colour going from light to dark.