Design wise, who has the best mega, and why is it Gyarados?

-Has completely different secondary typings with different implications
-More useful abilities pre-mega
-Flexible
-Mind games
-Solid mon
-Great mouth

I fucking love having mega Gyarados and not using it. I'll save my intimidate Gyarados to cripple mon throughout the game on switch in until just the right moment. I can setup as much as I like as regular gyarados and still have the option to sweep or continue to cripple with intimidate. Or, if I'm against an electric or rock type and fear getting hit, just mega and get rid of that Flying type. Likewise, if I'm against a bug or fighting type, I know not to mega evolve and resist with that Flying type. Both regular and Mega forms just facilitate each other so well.