Probably Proxy. Not easily by any means, but he rivaled low-level Galen somehow, and Shaak Ti didn't attempt to continue the fight after ragdolling Galen on Felucia, and instead committed suicide by jumping into Saarlac.
On the other hand, Proxy may not survive the Force burst that ragdolled Galen, but if he does, Shaak will probably fail seeing she preferred suicide instead of fighting Galen in sabers.
Proxy was capable of keeping up the Holocaust image of Obi-Wan for a second after being Shiak'd into the chest, but other than that I don't know about his durability.
Originally posted by Q99
I'd go Shaak. As high-end as he is, Proxy is still a droid, without the force. He was used in training Galen, but Galen wins those.
To be fair he did give near prime Galen a good fight when powered by the Core on Raxus Prime. Though Galen didn't want to permanently damage him and he did note PROXY was unable to pose as much of a threat as REAL masters.