By my estimation, the best creature in any given set usually falls into one of three main categories. It is either at a bargain cost for the size (such as Tarmogoyf or Isamaru), a card advantage machine (such as Dark Confidant or Ohran Viper) or it has something special and new that had not been seen before (such as Troll Ascetic or Doran).
Another thing that these cards all have in common is thier ability to avoid common destruction spells by either having high toughness or by being black, avoiding Terror-like effects.
The last common demoninator is that they are all cheap. Not counting the 4/4s for 4 that always seem to see a lot of play (Loxodon Hierarch, Chameleon Colossus, etc.), the most popular creatures are usually between 1 and 3 mana. Sorry, Siege-Gang Commander.
What does that mean for Shadowmoor?
I believe it means that this has a chance to be the best creature in the set:

For those of you that didn't take French in high school (which includes me*) allow me to translate:
Sygg, River Cutthroat (U/B)(U/B)
Legendary Creature - Merfolk Rogue
At end of turn, if an opponent has lost 3 or more life this turn, you may draw a card. (Damage causes loss of life.)
1/3
Why do I think that 'evil Sygg' will be the best creature in the set? Let's go through the check list:
Bargain cost for the size?
1/3 for 2 isn't going to break any speeding violations, but it's also not terribly slow. Combo with Doran if you want more bang for your buck.
Card advantage machine?
Certainly has the possibility, especially in Merfolk or Rogue decks in Type 2 and block (you did notice that he is a Rogue, didn't you?).
Something special and new that had not been seen before?
I've never seen this kind of thing done before, so I certainly think Sygg fits in.
What about avoiding common destruction spells?
Can't Shriekmaw 'em. Can't Tarfire 'em. Doesn't die to lowly 1/x or 2/x dudes. I'd say that's good enough for my tastes.
I think that Sygg has an especially big chance to make a splash in Extended. He might as well read, "If your opponent has a Dark Confidant or is playing Ravnica shock lands, win the game."
Let me know what you think.
Reubs
*Yo hablo espanol.
Tags: Tmp Shadowmoor Sygg River Cutthroat
Related to:
Magic: the Gathering