Not read through everyone else's contributions yet, but the following have all provided me with experiences close to epiphanies in that particular state of mind:
Eskimo - The Residents
Igor Wakhevitch - Logos & Docteur Faust
Eno/Byrne - My Life In The Bush Of Ghosts
Negativland - Escape from Noise
Amon Duul II - Phallus Dei
A common thread is that for me there needs to be a mixture of profoundly disturbing weirdness and bonkers silliness for it to really take me to the brink of utterly freaked out. The whole Heaven/Hell dialectic.