Probably the most famous (and one of the more sought after) prototype coins is the Jedi Knight coin, also known as the 63rd coin.  This is the finalized gold version of the coin, and is much rarer than it's silver counterpart (rare in its own right), in that only a small handful exist of these.  This coin was to be a mail away offer on the 93 cardback, also including a coin collector's album which also was never produced.  The coin is probably the most striking design, and definitely would have been the crown jewel on the POTF coin set.