CP... Tell you what, my answer would have been 3 years ago, we went Sept 07 and I have never seen the parks that empty. We walked onto majority of rides, i think Pirates at one point was a 20 minute wait and that was our longest of the trip, only because it was a Saturday.
However, we attended our first Mickey's Not So Scary Halloween Party this past Fall. What everyone one these boards says is true.......After the 1st parade and the fireworks show are through....MOST of the people take their tired kids, and head out of there. Then, during the 2nd parade, the remaining people are lined up for said parade leaving the rest of the park virtually empty. We walked onto Splash, got off, walked on again with nobody else in sight. Same for Pirates, Big Thunder. Monsters Inc maybe 20 of us in the theater.
So to answer your question, the Halloween Party after the 1st parade and the fireworks.