Good: D-Balls: The balls made for ball slams, virtually indestructible. Can also be used for other medicine ball exercises including wall ball and throws.
Best: Rogue Mk 14 D-balls: A Rogue exclusive, 14" D-Balls that are excellent for both ball slams and wall ball.
Best: Dynamax: The perfect medicine ball for everything other than ball slams, more padded than D-Balls and perfect for use in high velocity movements. Will not hold up to ball slams.