Viridian Longbow - When a select few started taking this over Bonesplitter in Amsterdam, many thought they'd lost their minds. Then it turned out they were almost definitely right. The Longbow combines a cheap casting cost with a huge effect on the game state, as your opponent's creatures with low toughness are suddenly facing death at all moments. Late in the game, it turns into a virtual machine gun as you move it from creature to creature, gunning down anything that moves. On top of all this, a single Longbow on your side of the board means that you have inevitability. If your opponent can't do anything to put the pressure on, you'll win the long game. It's that simple.
Sun Droplet - Everyone knows that life gain isn't good. Even Nourish, which is absurdly efficient at six life for two mana (at instant speed no less), hasn't seen play in any format. These assumptions led most to initially discount Sun Droplet as just another piece of garbage. As time went on, however, people started trying the card and discovered just how effective it is at making damage races impossible for your opponent.
Nim Lasher- This X/1 creature has the potential to grow very large, very quickly, but his lack of evasion combined with his fragility meant that few bothered to put him in their deck. All of a sudden the Canadians popularized the Lashdraft strategy, which drafts Nim Lashers highly and plays them along with high numbers of cheap artifacts. Slagwurm Armor and Neurok Hoversail both come out for one mana, which already helps out your Nims, but equipping these toys creates creatures that are downright insane. It's not uncommon for these decks to pump out multiple 8/7 fliers in a single game.
I walked around and talked to some of the Pros about what they feel are the current underrated cards. Answers below:
Antonino De Rosa feels that people don't value Leonin Bola nearly highly enough, because it shuts down opposing equipment, it shuts down their best guy every turn, and on top of all that it's cheap to cast and equip. On offense, it can even tap down multiple large blockers, clearing the way for your large attackers to face off against their little guys. He also gave an honorable mention to Talon of Pain.
Gabriel Nassif thought that Arcane Spyglass is a little underappreciated, although he doesn't think it should go in every deck. Still, in the right deck, it can "win the game for you easily." He also felt that the color Green is undervalued right now, although he seems to be one of its few supporters.
Terry Tsang gave the nod to Screams from Within, noting that he'd just picked one up eighth pick. The card can be absolutely insane sometimes, clearing the board of opposing X/1s and then landing on one of their other threats.
Marco Blume feels that Pearl Shard has always been underrated. The ability to prevent two damage for only one mana makes combat and damage races difficult for the opponent.
Paul Rietzl went with Loxodon Mystic, which is a card that has been constantly rising in stock for me as well. "People think it's too expensive but they don't know how to draft White," he said. "The best hand in Magic is Raise the Alarm, Blinding Beam, Loxodon Mystic, three lands, and any other card."
Olivier Ruel told me some stories about $400 taxi rides and expedited passport creation, but when faced with the question he couldn't come up with a good answer. "Probably something I underrate myself," he finally said.
Finally, Ben Seck felt that the most underrated was Lumengrid Sentinel. "It's always been great for me," he added.
There's a lot of variation here, so it seems like no one has the format completely figured out. Will there be any stars of this Pro Tour? Viridian Longbow set the precedent in Amsterdam, but we'll have to wait and see if any card can match that here.