Which Dragons of Tarkir card do you think will have the biggest impact on Standard?
Lee Shi Tian, Back-to-Back Pro Tour Top 8s this Season: Sidisi, Undead Vizier. It means midrange decks can have better access to their silver bullets and trumps.
Ari Lax, Pro Tour Khans of Tarkir Champion: Thunderbreak Regent. There are other cards worth mentioning, but that's the one which defines the entire metagame.
Shuhei Nakamura, Five Pro Tour Top 8s: Dragonlord Ojutai. The card is great. It's hard to remove when you play it, and easy to protect after that.
Owen Turtenwald, Chasing his Second Player of the Year Title: Dromoka's Command. It seems like a pretty safe answer. It's a popular, exciting spell and all of its modes are relevant.
Shahar Shenhar, World Champion Two Years Running: Dragonlord Ojutai. I think any deck with access to blue and white will want it, and Blue-Black Control may splash for it. It conveniently survives a Crux of Fate.
Patrick Chapin, Pro Tour Journey Into Nyx Champion: Thunderbreak Regent. It's a very efficient creature and exactly what a number of decks were looking for.