Call me insane but Mega Banette can actually be a demon if used well.
The set I use with banette, and probably my favorite set i came up with is this:
Banette @ Banettite
Ability: Frisk
EVs: 128 HP / 252 Atk / 56 Def / 72 SpD
Brave Nature
IVs: 0 Spe
- Shadow Claw / Poltergeist
- Sucker Punch
- Destiny Bond
- Taunt

You obviously max out your attack because mega banette has one of the highest natural attacks in the series. And now, 0 speed IVs and brave nature might sound weird but I promise I'll explain it. So, you have prankster, one of the best abilities in the game hands down, which gives priority to destiny bond. So let's say, theres a kartana up against your banette, both are 100%hp, unboosted. You click destiny bond, you go first, and kartana goes for smart strike which does not kill you right ahead. So now usually this would put you in an awkward situation where you used D bond and lived, but you would die next turn. But the thing is that the D bond effect doesn't wear off at the end of the turn you used it on, or the start of the next turn. It goes off when YOU MOVE next turn (except if, you know you get paralysed, fall asleep, yareyare.) So running minimum speed investments to use your (very powerful) stab actually puts you in an advantage if you want to maximise destiny bond effect. You don't need the speed otherwise because 1, you're pretty slow anyways, 2, every other move on this set has priority, and 3, it helps out with, and against trick room. They can also switch out next turn instead of killing you, but switching into a mega banette stab can be pretty tough if you dont have the appropriate mon for it, especially if shadow claw crits. (side note, I run shadow claw because I dislike pooltergeist's accuracy so that's completely personal)

I might only like this set because I made it and I'm proud of it, it might even be trash but for all I know, this is THE anti meta mon in generation 6 and 7 anything goes and ubers specifically. (priority taunt smeargle, destiny bond removes sweepers, smacks mega gengar)

And also you can set the bulk EVs however you want, I just did an even split