I'll start with this one:

CCS Ep. 7: Sakura gets ready to seal The Thunder
That's one that for me is tangled up in my early reaction to Cardcaptor Sakura, which I came to love and still love (Clear Cards not so much...). It was a classic moment in one of the early costumes, wearing a costume that has kawaii connotations, but in with an intensity that is anything but pink and cute. I'd wanted a cel from this particular cut, and I'd tried for a couple of them unsuccessfully. Then, in my first weeks of access to Yahoo Japan (thanks to Shopping Mall Japan), I saw this one -- the strongest image of the whole sequence -- come up, and at a time when I had some extra cash.
When the auction finished up, I was in the depths of a nasty winter cold, but it wasn't severe enough to make me forget when it was time to get in and bid my max. After I confirmed that I was the winning bidder I went back to bed and my meds. Nothing could prepare me, however, for when I opened my SMJ shipment a month later and saw how intense that Look really was. I've spent more for other items, and gotten some that are much scarcer and (arguably) more valuable as animation art. But this one remains the one that gives me more shivers than any other.