Which dragon should you put at the Center of your army in Dragonfire?

At the Center you put the dragon whose aura helps the whole team most, because that's where it spreads from. Depending on your goal:

• Long, defensive, attrition fights → a Champion. That's the breed with the best auras on average. The star is Rhysarion with Ebbing Fury (−27.5% enemy damage) plus +45 Recovery per round. Ideal for surviving 10 rounds and grinding the opposition down.
• You want to strike first → a Warrior, for its Initiative aura (Battle Rush: +Initiative to allies). Hitting first means stripping troops before the answer comes.
• Short burst fights → a dragon with big raw burst or one that amplifies a damage type, rather than Rhysarion, whose −27.5% throttles your own output.

The rule: never waste the Center on a plain bruiser with no useful aura. The Center is your team multiplier, not one more attack slot.