The best sand sifting goby I have had has been a Diamond Goby. Munched on sand 24/7.
http://www.liveaquaria.com/product/p...cfm?pCatId=220
I have also had experience with a yellow headed sleeper, he also sifted very good but had a bad habbit of swimming up while sand was still coming out of his gills covering my precious corals.