Creeeeeeeepy it certainly is, but I felt it was kinda like a shot at the Top 40 next to Tilt 
Everything by Sleepytime Gorilla Museum
I have a coworker who often gives me extreme MP3s to listen to, and he has given me Black One. If you like that, try Things Viral by Khanate. (The band shares a member with Sunn O))).) All twelve of the reviews on Amazon give it five stars; one of them calls it “doom metal minus the metal”. The most extreme thing he’s given me that I’ve enjoyed is probably either Sleep’s Holy Mountain by Sleep or As Heaven Turns to Ash by Warhorse.
It’s probably been twenty years or more since I listened to it, but somewhere in my stack of vinyl I still have an old copy of the Fugs “Golden Filth: Live At The Fillmore East”.
Most extreme - Negativland “Escape from Noise” (samples a Mr. Magoo cartoon, among other really odd things.
Least commercial - Wayne Butane - “Seduckted”. Don’t know anything about the artist, except a gal I once dated thinks she knew him, once, as a DJ in a strip club. He seems to have a vendetta going against Johnny Mathis.
The People’s Temple Gospel Choir Album- a CD with a really lame Gospel album they did in the early 70’s, and then the last cut is the tape of the Mass Suicide.
I’ve listened to it TWICE. The music part is bad & the “Finale’” is creepy as Hell.
I actually listen occasionally to Manson’s LIE- I hate to say it, but he really wasn’t a bad singer/songwriter/musician. If it weren’t for that pesky homicidal streak, he’d be a major seller in Europe & probably doing 3 AM Gospel broadcasts instead of Gene Scott or Arnold Murray.
That’d be Doktors 4 Bob, Bob. A Lewd Spectacle of Wanton Depravity. Both difficult to listen to and resolutely non-commercial lyrically.
Do I listen to it? Not really, not anymore. But I did play drums on a cover of “Half-Ass Drunk” from it once, as the last song of a set when we’d run through everything else we knew. Not that I play drums. But I’d already played guitar and bass, and we sort of worked out that everybody had to play something they don’t really play on that one, except the guy who came out of the audience to play guitar.
I have that performance on CD as well. <shudder>
“Aggressively non-commercial?”
Well, I don’t buy music to shock anymore (that’s so 1987) 
, but I’m not sure how commercial Israel Kamakawiwo’ole’s stuff was… sure, Facing Future was the best-selling Hawai’ian album of all time, but I wonder how many people only listened to “Over the Rainbow/What a Wonderful World” and missed out on the sweet voice and awesome ukulele on the rest of the tracks…
I miss Iz.
I have a bunch of foreign language cast CDS. Joseph in Italian? Got it! Jesus Christ Superstar in Russian? Got it. By Jeeves in German? Here it is. What about Chess? German or Swedish? The Beautiful Game? German or Hungarian?
The Holy Modal Rounders first two eponymous LP’s are/were classic examples of psychedelic old-timey. The Moray Eels Eat the Holy Modal Rounders is weirder–but has some gems. I may have listened to Indian War Whoop. Once.
The Red Krayola’s Parable of Arable Land was plenty weird but had at least one good tune. God Bless the Red Krayola & All Who Sail With It was just too obscure. But they were local heroes back in the day. And the second LP might be worth money–since it *isn’t * scratched to shit.
I’ve got 3 Nordic Roots samplers. The bands are pretty obscure outside the Scandinavian/Scandinavian-American folk-rock scene, but the music isn’t really too “extreme.” However–each CD is $4.98 at Amazon & they’re still available. (This is a recommendation!)
Tom Hamilton - London Fix
thats up there in terms of accessibility
Kilvert’s Pagan, meet John Lennon (and Yoko Ono)'s Unfinished Music #2: Life with the Lions
Wow. Yep - that one would qualify.
No Pussyfooting- Fripp & Eno
Actually Two Virgins is closer to sounding like a whole album’s worth of “Revolution 9.” Only not nearly as good.
I own probably the majority of the albums mentioned in this thread, as well as too many noise/free improvisation/electroacoustic music albums to mention, and the most forbidding would have to be either something by John Zorn/Naked City (with so many albums I don’t know which one to pick) or Diamanda Galas (probably Litanies of Satan). Oh, and then there’s Jandek. I only have two albums, but his whole vast discography is supposed to be uniformly ultra-weird.
A few namedrops: Iannis Xenakis, Derek Bailey/Music Improvisation Company, Fred Frith & Henry Kaiser, Karlheinz Stockhausen (I have a huge amount of his stuff), Pierre Henry (ditto), Morton Subotnick, Faust, Tod Machover.
I dumped most of my really out there stuff: Litanies of Satan by Diamanda Galas, Aloha! by Henry Kaiser. I still have all my Residents albums, though.
This is what I came in to mention… it’s very weird. I bought it, and listened to it once. It’s just him making random noises with his mouth, in various hotel rooms… very bizarre…