Paulo Vitor Damo da Rosa/Willy Edel/Allison Abe
da Rosa: Arbor Colossus. It was in my deck yesterday, and I didn't have many ways to deal with fliers. Plus it's just an incredibly powerful card in its own right.
Edel: Mountain? I did play red in both decks... I'd probably say Xenagos, the Reveler. He let me simply steal some games against opponents yesterday.
Abe: Prognostic Sphinx. It's so hard to deal with.
Armando Bulnes/Stephen Berrios/Ian Farnung
Bulnes: Revel of the Fallen God. My deck yesterday was an 18-land, 22 mana source, ramp into double Revel special. I think the over/under of the number of Satyrs I've made this weekend is 42.5. In this deck, it's probably Keranos, God of Storms. Though if someone had their way, it would have been in my sideboard...
Farnung: Observant Alseid. The card's just really powerful in a focused heroic deck. All of the Theros bestow creatures are.
Lucas Esper Berthoud/Paulo Ricardo Cortez/Walter Perez
Berthoud: This is going to sound really stupid, but it's probably Cyclops of One-Eyed Pass. My deck yesterday had so much removal that people couldn't keep creatures in play against me. I just needed a creature that could kill them quickly, and the Cyclops filled that role perfectly.
Perez: Any of the two-casting cost mana creatures: Golden Hind, Kiora's Follower, Voyaging Satyr, and even Font of Fertility. I have had a bunch of them between my two decks, and they have been the key. It's very important to ramp up to the four-drop slot in this format.