AMAROK by Mike Oldfield. Not just the music itself (it's quite a trip), but the story of its development:

He was under contract for Virgin to produce another long form album, but he and Branson had a falling out. Regardless, Branson threatened legal trouble if Oldfield didn't go through with it. He was hoping for a record with radio-friendly hits like "Shadow on the Wall" and "Moonlight Shadow" that could be peddled to radio stations.

So Oldfield composed AMAROK. It contains one single track, spanning the entire length of the 60+ minute album. That way, Branson couldn't simply lift any tracks and use them as singles.

But wait, there's more! Since Oldfield knew that if he did this, Branson would simply make a selection of excerpts from the album and use those as singles. This had been done before while they still got along or even before he signed with Virgin, so they knew it could work, cfr the "Incantations", "Ommadawn", "Hergest Ridge", ...

The "Amarok" track is... a special kind of composition. It's by no means bad - it's actually pretty great from a technical standpoint. But what you decidedly cannot do, is attribute it to any specific genre, nor easily mark the beginnings and endings of the different "tracks" comprising the record.

Thus, Oldfield won this battle and Amarok was pretty much impossible to use for radioplay. I still enjoy it to this day though.