Have two:
One I built a civilization built on ancient Egypt...but set in a swamp. Very much like the Nile delta but near mountains instead of desert (bit like bangladesh). Same flood cycle etc. Same focus on preservation (now as way to contrast the swamp instead of similar to desert). Egyptian Blue dye became green. Animal headed deities? Check. Replace pyramids with domes and helix shapes over obelisks. Used Egyptian style food and medicine. And replace ancient Egyptian/coptic with phoentian/carthage/hittite similar names. But the moment that made it all? The look on the face of a couple players with ancient history/archeology (one each) when they put it together and realized what I done and that they had missed it for two/three months.

Actually my other favorite worldbuilding moment was as a player. In ravenloft I rolled up a stupid-lucky character (and so made him a Paladin to knock him down a bit)...but my DM asked me to write up my family and hometown. So I did. All 600 locals were given a basic mapping (only 60 in the village itself-it was a farming region) and the breakdown of the family with tons of take it or leave it hooks for the DM. He ended up using the town twice more (with me) in other campaigns as a major setting. And from what i heard still was ten years later when I lost track of him.