One of the greatest still must be the unique physics of Thelan-4. I don't understand it myself, and don't care to, but the sight of the dim single photons falling slowly down from the sky and collecting in the rough valleys until they coalesce into near-blinding drifts is unforgettable. The way waves of iridescence will blow across the mostly-white banks, and the way colors will set in suddenly across an entire meadow of light - it lives in my mind to this day.