I own a pretty varied stash, still with some diapers I bought 10 years ago in it, and lately I've been saving a few, or possibly an entire pack *plus* a few spares, of the ones that aren't produced anymore or have undergone major design changes. Those, I usually keep with no intention of using them at all. I also have a good stock of discontinued models such as the plastic backed Delta M3 by Abena, or the plastic backed Euron Slip, that I stocked up on when I got news that they were being switched to cloth backed, and I'm going through them very, very slowly, until I'll reach that point where I'll have a couple of packs and a bunch left and I'll put those away. The oldest diapers I have around are a gift from a friend who's also into collecting them and they are a few tape-up American youth diapers from the early '90s (Attends I think?) which have a kind of plastic backing so smooth and thick that would be pure bliss to find on a diaper nowadays.
I also have a "specialties" box where I keep diapers like Dry 24/7s and ABUs, which I normally buy when I'm in the U.S. and that, however available in Europe too, would be more expensive to get here, and I therefore use very sparingly. In that box I also keep samples of diapers like DCs, Fabines, or Bambinos, that have been awaiting the right moment to be tested out properly.