Mine is the 1989 Robbie McCormack Buttercup card ... I've heard it exists in very small quantities, but never seen it.
2006 Danny Buderus signature tazo is a close second, that damn thing is the bane of my life.
Yeah the signed Tazo's would have to be up there, definatley not seen 3 of the St George ones, I doubt they ever made it out of the factory.