It comes down to what you're looking for. All three of Banettes ablities have usage in battle for various purposes even if you intend to Mega Evolve it.
Insomnia: Allows Banette to act as a sleep absorber, taking on that pesky condition to keep it off of your other Pokemon.
Frisk: Allows you to scout the opponent's item, which can be highly useful for identifying their strategy.
Cursed Body: Allows you to lock an opponent's move. It has its uses but is limited when you Mega Evolve as you probably get one usage out of it.
Out of the three, I'd say that either Insomnia or Frisk would be the better choice as Cursed Body has limited usage with Mega Evolving.
It really comes down to what you're after and if you want an answer about the abilities you have suggested, I'd go with: