Quoted from pinballinreno:

Never use chrome on pinballs.
Never use any type of plating.
It flat out won't wear well enough.
Use a 1 1/16" highly polished lower magnetism carbon steel ball.
It will be good for all applications.
Change the balls on modern games every 350 to 500 plays.
Wax the playfield with blitz or p21s pure carnauba.
More expensive balls will cause you to not throw them away as often as is needed.
Buy good quality balls from a good seller.