Zyrada

If we're going purely by quantity, Bull of Heaven wins, no contest.

As for music that's long that I've actually listened to and care about as more than novelties, here's a start:
Jacob Kirkegaard - Labyrinthitis
Louis Andriessen - De Staat
Machinefabriek - Singel
Earthless - Sonic Prayer
of Montreal - The Hopeless Opus or the Great Battle of the Unfriendly Ridiculous

I could've included more, but a lot of it was ambient, drone, or noise, and I wanted to have a little bit of variety. Kirkegaard and Machinefabriek are pretty clearly drone/ambient, but the others, not so much. It's a tricky thing when you get up into the 15+ minute range on a single track, because unless you're working in genres with a deliberately protracted style, it's hard to write something that holds up that long. Either your ideas get tired out or you insert so many to keep the momentum going that the end product is arguably a vignette of smaller forms that could be divided up into smaller sections and still have the same effect. (And yes, I'm talking about at least one of the tracks I listed.)

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