Im assuming that new exciting error coins still appear and take the crown (or do they ?) currently whats considered the error collectors holy grail ? Im both interested in your comments and gleaning some valued info.
Probably 1955 DDO, 1943 copper and, 1969 S DDO Lincoln cents. There's a bunch of them. I say this not as most valuable, but as some of the most iconic error coins that even non collectors would know about. Hard to give the title to just a single error though, since there's such a wide variety. Some error collectors like different things.
Holy Grail for me would be a non-gold coin struck on gold stock. There are a few Lincoln cents, indian head cents and a damaged buffalo nickel.