Their pre-Twoism stuff that was leaked mysteriously, and even if it wasn't Boards (Word of God implies they're legit, however), they're pretty awesome.
Tomorrow's Harvest is starting to be considered this by some of the fandom, with its epic, expansive sound inspired by 70s and 80s film scores.
The Alternate Reality Game leading up to its release also qualifies, and is arguably the most elaborate advertising campaign for any media, involving rare vinyls, hidden YouTube videos, and hidden codes which led people to a website which, when all 36 codes were input, revealed the title and release date.
"Roygbiv", their arguable Signature Song, is so well-loved that when Warp Records announced a 20th Anniversary box set, they let the fans choose a few of the songs that would appear in one of the CDs. "Roygbiv" won second place, only suffering by a landslide defeat from Aphex Twin's "Windowlicker".
