At the top tables at the beginning of Day 2, the most popular options were Green-White Tokens and Naya Midrange. This would imply that Sylvan Advocate and Gideon, Ally of Zendikar are the key cards to beat in the current Standard format.
You can find the full breakdown of the archetypes of the Top 100 players going into Day 2 (i.e., the Top 100 players on the Round 9 standings) below.
|Sultai Seasons Past||2|
|Abzan Seasons Past||1|
|White Weenie Humans||1|
So Green-White Tokens and Naya Midrange are the most popular decks, followed by a variety of other archetypes. Looking a bit more closely, we can find a number of commonalities among those other decks.
First, Collected Company and Eldrazi Displacer had big followings, and they were often seen in the same deck. Both are key pieces of most Cryptolith Rite decks, and Eldrazi Displacer has become a frequent inclusion in Bant Company decks as well.
Second, Languish is the go-to card for control players. It doesn't matter whether it's white-black, Sultai, Abzan, black-green, or Esper—Languish appears to be the sweeper of choice, frequently flanked by Transgress the Mind and Ultimate Price.
Finally, Thalia's Lieutenant is the card that aggro players flock to in the current Standard. Bant Humans may not be as hyper-aggressive as White Weenie Humans, but for both decks, Thalia's Lieutenant enables the fastest starts in Standard.