Most important word class rarity seen? Like others I saw the British Guiana 1 cent black on magenta of 1856, at Interphil I believe.
It stimulated my imagination as a young child collector, but by the time I actually saw it, it didn't do much for me.
Far more interesting to me was the material in the Rudolf Wunderlich exhibit at that same show, which I photographed in the frames and spent hours notating and studying. I had travelled to Philadelphia for my first international precisely because of this exhibit. Now, one of the items from that exhibit is in my collection. It is the original pencil and wash model for the proposed ten cent of 1870. It is unique.

I have several items which are unique, but this one is more philatelically significant than the others, since it is part of the earliest stage of development of a new series of postage stamps.
Seeing it in this scan I am really tempted to clean off some of the fingerprints and such it has accumulated over the years. They are not part of its original condition.