Gunnar, Adley, Holliday. 50/50 on Burnes.
Gunnar because he's the best player, probably the most valuable. Adley #2 because things turned around here when he came up, he's probably the face of the franchise, and is growing into the undisputed leader of the team. I don't know if he has another level to unlock yet, but I also believe his importance to the team over the past two years can't be simply calculated in WAR and it won't be measurable moving forward by advanced stats.
Holliday 3rd, and that's just because he's the newest. Arguably the most talented of the group.
Burnes, like I said in the other thread, it just depends.