What are the most radical/experimental songs or tracks done by a pop or rock band in your opinion?

Orchestral Manoeuvres in the Dark ABC Auto Industry:

 

Deceptively experimental

 

Obviously, not many here would consider Negativland to be a 'pop/rock' band, per se.....
though, they HAVE written 'pop/rock' songs, the majority of their work is stuff like this!
One of the few bands I''ll follow to the gates of hell!

 

Rock and pop are obviously not avant garde forms, and rarely -- including in most instances posted in this thread -- do you encounter anything radically, to refer to the title of this thread, outside musical convention, and especially not among "songs" -- if it's a song, it's probably not all that radically unconventionally. But I'd go with Metal Machine Music.

 

Suzanne Vega's 99.9F°, produced by then-husband Mitchell Froom, was an unexpected but brilliant swerve into electronic/industrial clang and quite a shock to many folk fans in 1992

 

Chicago - Free Form Guitar

 

Lumpy Gravy

 

Most anything by the UK 70s group Henry Cow could qualify, but my favorite is this recording from 1978:

 

This Bo Diddley track prefigures about 60% of Derek Bailey's approach.

 

Marvin Gaye's "What's Goin' On" Album

 

Almost the entire subgenre of prog, avant-prog, will fit into this category.

But then, it might have a lot to do with how far you want to stretch the meaning of the term "rock".

As @mx20 mentioned above, Henry Cow is a good example. But from there, the list of bands in this subgenre is long, and experimental.

Univers Zero (Belgium) - Dense

 

Hell yes. Here’s 4/8/72.

 

Art Zoyd (France)