As a physicist, I've always been partial to Maxwell's work -- he deduced the induction of a curled magnetic field by a changing electric field solely from mathematical considerations, and from this, was able to guess the nature of light before any other human.

I've mixed feelings about Descartes. The pull of the Cartesian Theater has muddling effects in serious cognitive philosophy. On the other hand, by making the concept explicit, he did make it easier for others to point out that it was wrong.