Holy crap. Takk... by Sigur Rós is definitely one of the weirdest albums I own. Aphex Twin's 26 Mixes for Cash is probably the king of that spot, but Takk... is pretty nuts too. I don't know what you could compare it to, so I guess it's, uh, the best of its kind. Whatever its kind is. It's full of ethereal babbling in a language I can't understand, and much of it sounds more like a sonic experiment of some kind than music.
In fact, despite the fact that all three have incredibly different sounds, I'd kind of lump Sigur Rós, Aphex Twin, and The Mars Volta together in their own category of pretentious insanity. I find it really hard to see past the fact that there's so much bizarre noise in the music produced by those three artists, and I tend to even like thick, noisy music. Amon Tobin is one of my favorite artists, and despite his music being comprised of a lot of bizarre noise too, I find his sound to be very different. I'm not sure why it is.
That said, Takk... isn't bad. Parts of it are grating, like the high-pitched ooooohing in so many of the tracks, and it's not something that I'm really in the mood to hear most of the time. I can't really recommend it to anyone. Check out the video I linked to at the beginning of this post, though. That song is an excellent example of what the rest of the CD is like.