My vote would go for Dunlop squash balls since they have a very low coefficient of restitution (they don't bounce much). A low coefficient means more kinetic energy (vibration) is lost as elastic energy and heat thus offering a degree of structural isolation.

I'm not sure why Superballs have found favour (not questioning their use) but from an engineering point of view it's a very bad choice because they behave in precisely the opposite manner ââ'¬â€œ hence very little isolation.