Most different?
I've never heard anything like the songs on the Cardiacs compilation CD that Lee_B sent around recently, and in a similar sorta way, Devin Townsend's "Infinity" is, with the exception of curious Cardiacs similarities, highly unique in all ways imaginable.
Ulver is truly a band like no other. Do you know anything remotely like "Themes from William Blake's..."? I know I don't.
Finally, Frank Zappa rules the notion of "different" from the perspective that not only do several of his albums sounds like nothing else ever made ("We're Only in it for the Money" comes to mind...) but his albums also sound nothing like each other! Freak Out, WOiiftM, Hot Rats, Burnt Weenie Sandwich, The Grand Wazoo, 200 Motels, Overnite Sensation, Roxy and Elsewhere, Joe's Garage, and The Yellow Shark (some random examples) all sound completely unique from each other from style to composition to production.
(and, he even wins out for the best song titles, beating even Anal Cunt, who made it a major part of their schtick. "Prelude to the Afternoon of a Sexually Aroused Gas Mask"? Beat that...)