Well, then, OK. Un-findable makes for a short thread, and a bit of a paradoxical exercise, but we can revisit your suggestion of Devil's Castle, by which I assume you mean Melies' Le Manoir du Diable from 1896. Two to three minutes of all manner (manor? manoir?) of mephistophelian mayhem, from bats to cauldrons to cavaliers with crosses.
There are a few seconds of it in Real Player at
http://www.mshepley.btinternet.co.uk/melies2.htm (click on the camera icon at end of paragraph about the film)
But yeah, no one has found the entire thing (or no French archive has coughed it up). So we can agree on
Le Manoir du Diable (1896)
Frankenstein (1910) and
London After Midnight (1927)? Any others?
I'll still hold out for Frankenstein. Which really isn't "un-findable" (see
http://www.filmthreat.com/Features.a...One.inc&Id=284 ). Nothing is truly "un-findable" unless it never existed in th first place. There's always a private collector or museum archive somewhere that seems to surface. And then the legal entanglements ensue.
As to what "classic" should topple and be supplanted by the new find? I never much thought Creature from the Black Lagoon deserved the same stature as the other "classic" monsters.