Here are my thoughts (I'm only considering standard playability)
Most format altering card (not counting the
shock lands):
Rhythm of The Wild-Rhythm of the Wild will end up being the best or most format altering card in my opinion.
-in a format with
Llanowar Elves, Rhythm is so powerful. The options of creatures that can follow it on turn 3 and 4 are
pure powerhouses.
-the synergy with
Growth-Chamber Guardian is pretty nasty. It gains the counter as it enters, therefore you search your library for another copy. You can then play the second copy that turn (turn 3) and search for another copy for your hand. The result is two creatures on board (totalling 6 power/toughness) and a third creature in your hand that allows you to do the same trick next turn if you want.
-it is fairly certain that U/W or U/W/
x control is going to be a major player in this standard, if so dropping a Rhythm before the opponent can counter it with a 3cmc counter may be game over for them.
-Rhythm may be such a problem for control that it forces players to change from draw-go to tap-out control.
-The truest sign of a "best card", is one that warps a format to respond to it
Cards to look out for:
1)
Bedevil would probably be the best card, however it is hard to cast and is color restrictive.
2)
Judith, the Scourge Diva may be the card to watch out for if an Aristocrats style strategy rises to be tier 1.
3)
Frilled Mystic is crazy powerful, however it is likely that the hardcore simic color restrictions holds it down from seeing a lot of play.