I'd say "Salo"....it's one of the few films to EVER leave a deep impression (or is that "mental scar"?) on me ("Happiness" and "In The Company Of Men" are the other two).  Yeah, "Porcile", "Gospel" and "Teorema" are all great...not too much into the trilogy, but "Salo" is so powerful and unforgiving that there's no way I could ever say any of his other ones are better...then again, that's just me...