The best looking ones are chrome acorn nuts. But you have to cinch them down tightly or they'll vibrate off. That gives the plastic no wiggle room at all. The nylons are about the same, but have a locknut quality that will usually prevent them from vibrating off if left a little loose to allow the plastic to float. Speednuts are faster, safer and can be (like nylon versions) painted or inked custom colors to match their surrounding area.

I think shattered plastics are pretty rare on games that have <50 volt flipper coils.

Here is probably a good a place as any to mention that installing appropriately sized fender washers under the head of post screws (not over the head contacting the plastic) will prevent the ball from striking from underneath and supports the plastic from below if the ball strikes it from above.