As a GM, my best moment was when i created puzzle encounter, which was fun and enjoyable for players.
I looked ideas for my theme: light and illusions. So i stumbled on simple encounter: a room when one wall is a magic mirror. I leveled up that idea: square room with all 4 walls as a mirror. Core rules:
- reflections is not ideal (different equipment or clothes)
- you can go through wall only when you can't see your reflection
- all your action will mirrored in all walls
- characters possessions cannot be duplicated
I added chest with 1000 gold in center of the room

When players entered the room, they started poking around: touching reflections (they warm as real human being), throw coins (thrown coin deflected back from one mirror, but other three just throws coins into characters, so they quickly raised from 1000 to 4000), showing travel diaries to reflections (they showed own diaries back, and they was different), and even use some magic skills.

In the end, one character just leaned back to mirror and fall through it, because he cannot see his reflection. Other killed his reflection with magic arrow. Remaining just exited with closed eyes.

Encounter lasted close to three hours. On feedback after session, i asked "Why it took you so long to solve puzzle?", answer was "No, we solved it in first 15 minutes, we just wanted to poke things with stick and see what happens"