Best Crosshair

There's no best setting for anything but general guidelines I'd follow are:
Use colours that stand out/don't blend in with backgrounds as easily.
Use a crosshair size that you can see and track easily


Other than that it's up to you. Also try sticking with settings for longer periods of time to get used to them. Switching too fast will not allow you to adjust to your new settings and do a fair amount of practice with new settings to see if it really is or isn't right for you however sometimes you can find that one setting that just clicks with you