I can't pick just one.
Here are 5.
1. Noon on a sunny day under a leafy tree that lets through many shafts of sunlight.
2. A particular 99 cent store. Somehow the long bars of fluorescent lights, running high above and wall to wall, just happened to be spaced such that my former 2.26 ct traditional asscher exploded into a shimmering rainbow of pastel hues.
3. Driving on the freeway at night, I'll hold my ring up to face the headlights behind me. Obviously it's safer to do this as a passenger than as the driver.
4. A certain concert hall with tiny halogen lights way up in the soaring black ceiling.
5. Outside on a bright but cloudy day. There is no fire, but the soft even light makes my Octavia's striking pattern is easiest to admire.