Martina's red clay records are consistant, but it does not impress me than her results with rebound ace. So I think its rebound ace. I think when she lived in Switzerland, she had all the surfaces built on her backyard particulary rebound ace where they imported the same stuff from Australia.
Martina plays well on any Surface. The results show mostly Rebound Ace:bounce: and Carpet. As u can see her best prefromances are usually in the couple of months the WTA tour emerges. Her results in Australia and Japan just show that shes very dominating on Carpet and Rebound ace.
As far as all round ability goes, there is not a player that comes close to her...She can play on any surface and be a major threat, no other current player can say the same. (or if they do or their fans, then they are kidding themselves).
Because she does have that ability on all surfaces, then her favorites would be the less punishing ones on the knees...maybe clay.