there's no "good" or best crosshair yo.
just use the one you feel more confortable and practice with that one.

SIZE MATTERS.

i always used to play in small
felt uncomfortable at first to change to large
but after i changed it to large
i felt my crosshair more stable than small one
i cant change to small now cuz i cant control its movement

In my opinion
640 is obviouly better
cuz it has less pixels
that means less pixel to search for the head
and less pixels for the crosshair to move while spraying
so it acquires same recoil pattern

thats what i think (may be wrong)
and sorry for my bad english :)
GL HF !!!!!!
cs 1.6 rox :)