Elevated
A 2009 4k intro with snowy mountain landscapes, still the go-to exhibit for how far 4096 bytes can stretch.
Elevated was made by RGBA and TBC — visuals by Iñigo "iq" Quilez, synth by Mentor — and won the 4k competition at Breakpoint 2009. For a few minutes the camera glides over procedurally generated peaks with fog, snow and low evening light, set to a moody soundtrack. All of it, music included, fits in a file smaller than plenty of favicons.
The trick is noise: the terrain is built from layer upon layer of mathematical noise functions computed in GPU shaders, so no geometry needs to be stored. It routinely lands near the top of best-of-the-scene lists, and iq carried the techniques over to Shadertoy and his widely read articles on procedural graphics.