You can, of course, get cool tones all over the place so, experimentation is key. But, to answer your question, I've seen Eric Johnson comment that his preferred place to pick for optimal tone is right behind the middle pickup.
I've experimented with this quite a bit. I've found that, even on other guitars, not just strats, that this is probably the best place to pick when taking tone into account...although I've also always liked to pick up close to the fretboard as well.