The 4 strongest are Ignia, Irene, Gildarts and Suzaku for me.
However this is a battle royal : since Ignia is battle crazed, I imagine he will release his most powerful flames all over the battlefield all at once. The 4 winners would then be Ignia and 3 who resisted longer than the others.
So to Irene, Gildarts and Suzaku who are clear choices, I would add another : Brandish (you didn't see that coming, did you
)
Indeed, Ignia is a rather small dragon compared to big dragons like Merc or Aldo. If Brandish went country size, yes her lower legs would probably be completely burned off but she would still survive. And she may endure serious Ignia longer than people who are stronger than her that way (since he's not focusing on her)
She could even make the ground below her country level high and completely escape Ignia's wrath.
So my 2 picks would be Ignia and Brandish, now I'm still not sure the 2 I'll pick between Irene, Gildarts and Suzaku for surviving Ignia longer.