Okay. Shadowmoor's main feature: allied colored hybrid and "double colorless hybrid". Secondary features: -1/-1 counters (as opposed to lorwyn's +1/+1 counters). Tertiary feature: color matters (as opposed to class matters). And then some stuff that shows the above: wither, persist and conspire. A good representative of the set should have (some of) these features, and it needs flavor.
So that leaves us with a couple of contenders: Wort, the Raidmother and Giantbaiting (because giants are cool right?). Then, some cards worth considering: Kitchen Finks (with flavor), Murderous Redcap (goblins used to be cool). There are more cards, the demigod cycle, the aura cycle and the hatchling cycle. The aura cycle and the demigod cycle are strong, the hatchling cycle not so much. The demigod cycle doesn't really showcase multiple of the set's features, and I don't feel the other two cycles are really appealing. Then there are more cards that do showcase some features (prominently hybrid mana... pfff) but aren't pushing the set's mechanics. Cards like Sygg. And some cards that are just downright cool, like Necroskitter and Valleymaker.
One thing though: Reaper king is not worthy of the hall. The card is unmistakably cool, but it offends one of the main themes of the set: it makes race matter. It's a funny exception in the set though.
So I'm nominating Wort, the raidmother. It's a tough choice between Wort and Giantbaiting, because I'd rather have Giantbaiting in a deck than Wort. But I think Wort has more appeal to a broader audience, and a legend is cool anyway.