Show me a cel that is dear to you and yet is completely unknown to other collectors.
I don't mean just a rare item, one of a very few cels or sketches from a well-known series. Or one from a series that has its fans, but isn't for some reason sought after by anime art collectors. Or something from an "under the radar" series that isn't as well known as it ought to be. I could choose something from Princess Tutu for the first of these, from Tonde Buurin for the second, or from Asatte no Houkou for the third. But none of these is really "completely unknown." So I pick:

Jack Oshinumi and Puck from Yōsei-ō, or "Lord of Faerie"
This cel is from a one-episode OVA produced by Madhouse in 1988. It's an adaptation of Irish myths and folktales about the culture hero Cúchulainn and the rich fairy lore including the trickster spirit Puck. (Lucifer and his sweet babe Queen Mab/Maeve also put in appearances.) It's based on a 5-episode 1978 manga by Ryōko Yamagishi that develops a prominent shōnen-ai theme featuring the mythic Irish hero and a bishy mortal from Hokkaido named Jack Oshinumi. (Being of Irish descent myself, I want to make that last name O'Shinumy).
That hauled me over short in one of my trawls through Mandarake's daily auctions in 2012. I knew not a thing about the OVA, but it was such a well-designed and brilliant cel that I counted the auction down and nabbed it. Then I tried to find out something about its context and background, and at that time there wasn't much to find out, though Yamagishi subsequently gained a reputation as a major, prize-winning mangaka. Eventually I tracked down a fan's English summary of the manga and made a good guess as to what the cel showed, which turned out to be pretty close to the mark when a version of the OVA (in raw Japanese) showed up on YouTube six years later.
I've never run across a mention of this OVA in English, and this is the only cel I've ever seen show up for sale. I do keep Yōsei-ō on my Yahoo Japan trap line, but only see used copies of the manga in the items returned. It's about as totally unknown a cel as I've ever chosen to own, and yet it continues to haul me over for a long, admiring look, both in my gallery and in my cel book.