Mega Gyarados is such more than you're making it out to be. It technically has three types, with two varying combos, if that makes sense. You can stay Gyarados as long as you want, so you're Water / Flying until you decide to become a Dark-type, that sounds weird, doesn't it? Basically, Gyarados can come in on say Conkeldurr, and Conkeldurr can try and use Drain Punch expecting the Dark-type change, but you stay Flying, making it a free set up, then you can safely boost / attack until the Pokemon is gone. Finally, you can mega evolve and you've now got a huge threat. Unaware cannot possible beat it thanks to Mold Breaker, and honestly, it doesn't need Crunch. I certainly wouldn't give up that sexy combo of Waterfall / Ice Fang / Earthquake, not for Crunch at least. Priority move? Why the hell does it need one anyways? There are 32 Pokemon (Gyarados included) in A--, A-, A+-, and S-rank that lack priority of use, and see where they are? I hardly count Priority as an argument unless saying a Pokemon is weak to priority. Here's all the priority in A-S rank:
252+ Atk Life Orb Aegislash-Blade Shadow Sneak vs. 4 HP / 0 Def Mega Gyarados: 48-57 (14.4 - 17.1%) -- possible 6HKO
252+ Atk Life Orb Bisharp Sucker Punch vs. 4 HP / 0 Def Mega Gyarados: 84-100 (25.3 - 30.1%) -- guaranteed 4HKO
252+ Atk Choice Band Huge Power Azumarill Aqua Jet vs. 4 HP / 0 Def Mega Gyarados: 55-66 (16.5 - 19.8%) -- possible 6HKO
252+ Atk Huge Power Mega Mawile Sucker Punch vs. 4 HP / 0 Def Mega Gyarados: 76-90 (22.8 - 27.1%) -- 54.8% chance to 4HKO
252 Atk Aerilate Mega Pinsir Quick Attack vs. 4 HP / 0 Def Mega Gyarados: 91-108 (27.4 - 32.5%) -- guaranteed 4HKO
252+ Atk Technician Mega Scizor Bullet Punch vs. 4 HP / 0 Def Mega Gyarados: 55-66 (16.5 - 19.8%) -- possible 6HKO
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 4 HP / 0 Def Mega Gyarados: 75-89 (22.5 - 26.8%) -- 31% chance to 4HKO
252+ Atk Choice Band Talonflame Brave Bird vs. 4 HP / 0 Def Mega Gyarados: 219-258 (65.9 - 77.7%) -- guaranteed 2HKO
+1 252+ Atk Dragonite Extreme Speed vs. 4 HP / 0 Def Mega Gyarados: 136-161 (40.9 - 48.4%) -- guaranteed 3HKO
252 Atk Choice Band Terrakion Quick Attack vs. 4 HP / 0 Def Mega Gyarados: 61-72 (18.3 - 21.6%) -- possible 5HKO
252+ Atk Mamoswine Ice Shard vs. 4 HP / 0 Def Mega Gyarados: 33-40 (9.9 - 12%) -- possible 9HKO
Here's all that priority, and only one priority stands out: Talonflame, which has 0 business staying in on M-Gyara. I fail to see how Mega Gyarados is not S-Rank.
Pls explain to me why Azumarill is Aqua Jetting the Gyarados instead of just OHKOing with Play Rough and Scizor isn't going for Superpower. Gyarados isn't S rank because it has a tough time in the current meta with the rising popularity of Ferrothorn, the omnipresence of Azumarill and Keldeo, and even Sash Breloom is having an effect on it. Not to mention the fact that Gyarados is NEVER at full health when it sets up. It is a good offensive check to Keldeo and Lando when it hasn't MEvolved and usually takes damage from one of them during the course of a battle. Not to mention its weakness to SR. I mean, it's not that Gyarados isn't good, but it's nowhere near the best Mega, and I don't think it deserves a spot in S with the most meta-defining Pokemon in the game.