Mine is definitely Kay Stopforth's Universe Cards, first edition. I don't own any others, and I've looked at many. None have the appeal of the UC.
I love outer space and astronomy, and the way Stopforth presents lessons inspired by visual and behavioural patterns of celestial phenomena, as a journey from our tiny planet out to the edge of the Universe, is fantastic. I prefer the simplicity of the unaltered photographs as opposed to the CGI-art enhanced second edition; the photographs carry mysticism in and of themselves.