Okay yeah. I'm sold. Mega Gyrados is the best.
It's so flexible. It'll still die to Talonflame in one hit, but ignoring that one for now let's look at what it can do.
1. In normal form, Gyrados is flying / water. Sure it has 4x weakness to electric, but it's also immune to earthquake as well as resistant to fighting. So if a prominent earthquake user comes out, he has nothing to fear at all and can just waterfall his enemy to death without reprecussion. In fact, it can probably set up a substitute as well. It can do the same with a fighting type, as fighting types don't do enough damage to break the substitute in general when he's in his regular form. Blaziken can't do jack shit to him. Well... it can try but it'll die doing so.
Similarly, if there's a big status user like Klefki or Malamar on the field, he can still just opt for substitute first to be completely immune to them.
2. Provided the enemy isn't going to use earthquake or perhaps they aren't using a fighting type, Gyrados can switch to Mega form and while it loses flying type for dark, it gains so much damage and tankiness than now his substitutes are so freaking difficult to remove. You must supereffective him to remove the substitute. If he's behind reflect, then good luck.
Now he can hit so hard with his amazing coverage. Ice fang, earthquake and waterfall can hurt almost everything in the game. He's also reasonably fast, and can outmove things like Mega Ampharos. But most of all, it's really his substitutes that become godlike.
It's really good. Highly recommend you try it.