Fractals FTW
I've always loved the mathematical beauty of fractals.
One of the first programs I installed on my original PC in the 90's was Fractint. I spent hours creating, colouring, and later animating fractals.
In the 2000's the Milkdrop visualizer for Winamp was my go to.
Recently I discovered Electric Sheep 4K available on the Apple TV. Though it cannot synch to music being played (like the old WinAmp) from Roon, it still is captivating when streaming music from the likes of Saul Stokes or Future Sound of London.