Tough choice...but I went with Brycen in the end because (assuming all 3 Gym Leaders' Pokemon kept their original movesets), here's how it may play out:
-Dragon Tail, the main Dragon STAB of Drayden/Iris' Pokemon, has negative priority, which means that Dragon Dance won't help. So that means the Dragons are forced to eat a strong Frost Breath (or Icicle Crash in Beartic's case) before they can strike back. However, Druddigon could have high enough Defense to survive and strike back with Revenge.
-However Brycen's Cryogonal's Defense is very low, so that means it'll fold to a Dragon Tail, assuming Druddigon, Fraxure or Haxorus survive its Frost Breath or Aurora Beam.
Solution (for Drayden and Iris): give all their Pokemon Dragon Claw or Outrage in addition to Dragon Tail so that they can hit hard without taking a fatal hit in the process.
Appearance-wise, though, I'm going with Drayden and Iris; Drayden has a cool beard and Iris has a much bigger role in the anime as one of Ash's companions.