>>53877790Miraidon is definitely better, but discounting Koraidon is silly. It effectively has 3 STABs due to Sun giving that 50% Fire boost. Dragon, Fighting, and Fire alone let it hit over half the meta game for SE damage, and another half for neutral. The only Pokemon that resist its 3 STABs are Azumarill, Tapu Fini, and Primarina. It's got good bulk, even for the Ubers tier, and its 135 speed tier coupled with its access to Flame Charge means that very little can outspeed it after switching in.
Zacian-C, at least, couldn't really punch past Fire types or Quagsire. Koraidon can punch past anything. The only thing holding Koraidon back is Flutter Mane.