I voted for House of 1000 Corpses, because it's demented and yet psychadelic, and was a fun experiment in bizarre cinema. Devil's Rejects is a close second, it's almost as far from Corpses as to be in a different genre, except with the same characters.
The HalloweeN films were fun to see what RZ could do with an old classic slasher. I actually liked the first one better than the new one, but I like that Zombie was willing to try something radical with it, rather than just another sequel. White horses? Strange and yet refreshingly different. And no Busta Rhymes in sight. Hopefully Zombie's The Blob remake will be as interesting.
Overall, I love RZ as a director/writer, and can't wait to see how his skill evolves further.