For years, mine was this gif I saw on UsVsTh3m in 2013. It had no context, and the only hint to its origin was the barely legible "29. 暑〜い!" overlay. I had a guess that this was from a Japanese TV talent show. I wanted to find the original video, but the trail went cold.
Years later, I don't know how, I heard of a series of Japanese TV specials called Kesou Taishou, known overseas as Masquerade. UsVsTh3m got bought by the Mirror and their post was taken down, but I still had the gif. My Google Translate research skills had improved, so I looked up Masquerade on Japanese Wikipedia and searched all the episode guides for 暑〜い!, and in the shows from 2007, I found it! It was entry 29 in the 78th special, and it won third place there.
I try not to post things that don't have a legal source, so I quietly updated my blog post and didn't mention it anywhere else. Until...
Three months ago, my partner started getting YouTube recommendations for this show. I checked, and the show's new official YouTube channel had just uploaded this performance a few days earlier. I made a better gif of it and posted it to !gifs@lemmy.world , and they liked it!
For years, this was my Internet white whale. I wrote about my search for this source here.