Its very hard to say.
Usually I use to say that the fastest rubber is the one the player feels the best, controls the best and plays the best with.
A well designed test may show "the fastest", but most probably it will be some kind of graph of the speed dynamics depending on the strength of impact, and it would be executed with flat hits.
Spin implementation may change the picture a lot. The dead no spin ball is more unstable and decelerates much faster than the top spinned one, and this is even more expressed with the new ball, which tends to go faster with the increase of the top spin.
So depending on the particular player style and top spin abilities, as well as the particular rubber system design, including many factors plus tangentional deformation and reaction, the results may be very different from an "objective test graph".