The z3, 42.5, is a nice workaround because it's not sensitive at all. It's very fast though...I would say shore O 45 (ESN scale) be the softest. But some considersation fit the top sheet should be taken. I.e Rakza 7 soft and Vega Europe are too soft.
Too soft rubbers would make serve receive and short game more difficult since the are usually more spin sensitive and catapulty.