imo simpler is better. You want it to be clearly visible at all times, but take up as little space as possible. much like any other HUD element basically.
crosshair3, crosshair5, crosshair6, crosshair7, brackets. All fine
If you're looking at custom ones you can consider outlines or shadows
Stuff like pilot crosshair or giant crosshairs just add clutter
If you go more obscure than a variation of a dot, cross or bracket you're probably overthinking things (and I say that as someone who has made and used some very questionable crosshairs in the past)