Eventually I would like to come back and add more talking about the different characteristics of balls and which are better for what purpose.
Many people ask about ball sizes, weights and stuff.
I think an acrylic, silicone or other heavy ball would be best. The weight is important. You want to really feel the ball set down into you hand. A tennis ball for example is too light, it bounces right off your arm. The added weight of a silicone, acrylic or metal ball helps establish a connection with the ball, so you when you move your arm, the ball moves too.