There are a good number of Empire Strikes Back (ESB) proof cards available to collectors today, second in numbers only to the 48-bk Revenge Proofs, and it is relatively easy to pick up an ESB proof card example. This is simply due to the number of characters which were released on Empire cards, and the number of different offers which featured on this cardback, each requiring a proof card.
Due to the shear number of character and cardback combinations, it is virtually impossible to give any kind of accurate heirarchy scale for the ESB proofs and sample cards.
50 characters were released during the line (although all are not known on ESB proof cards) including both versions of C-3P0 and R2-D2. 5 different card front types were released during the ESB period and multiples of most cards exist.
The 47 and 48-back proofs are probably the hardest examples of ESB proof cards to find, with only one of each character known to exist (and by known means are all 47/48 characters known).
The Empire Strikes Back Proof Card Types:
More ESB Proofs can be seen in The Empire Strikes Back Gallery