What’s the best way to clean a tempered-glass PC side panel without streaks?

A normal glass cleaner should be fine for a bare tempered-glass panel. Spray it onto a microfiber cloth rather than directly on the panel, then wipe it dry with a clean section of the cloth to prevent streaks.

Glasses-cleaning wipes work really well and generally don’t leave marks. If you use isopropyl alcohol, apply a small amount to the cloth instead of spraying the panel, and keep it away from the electronics inside the case.

Plain distilled water and a clean microfiber cloth are usually enough, especially if the panel is only dusty or has fingerprints. It leaves less residue than many glass cleaners.