HELLO CHILDREN.
I'm Banette, #354, and I have:
Type: I'm Banette, #354, and I have:


Ability: Insomnia / Frisk / Cursed Body (Hidden Ability)
Base Stats: 64 / 115 / 65 / 83 / 63 / 65
...and when I'm given a stone of everlasting power (okay well actually it lasts till I die, but whatever), I attain much, much, more scary magic:


Ability: Prankster
Base Stats: 64 / 165 / 75 / 93 / 83 / 75
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Notable Moves:
Calm Mind
Confuse Ray
Destiny Bond
Disable
Gunk Shot
Hidden Power
Imprison
Pursuit
Shadow Ball
Shadow Claw
Shadow Sneak
Substitute
Sucker Punch
Taunt
Thunder
Thunder Wave
Thunderbolt
Will-O-Wisp
General Analysis:
Now that I've stopped speaking in Banettespeak: quite frankly, the only reason you'd want to use Banette is because of its Mega Evolution. Yes, while this does take up your single Mega Slot, Mega Banette is dangerous by itself for good reasons. The most notable changes it receives after evolving is a giant increase in Attack, in addition to the much-welcome Prankster ability. Small changes include an increase to its Special Attack, making special attacks much more usable, and defenses, though they're not too fantastic.
So now the question is: why would you want to use this thing? Admittedly even to me, the answer is quite hard to come up with, seeing as a proper metagame doesn't even exist at the moment. However, looking at its stats, movepool, ability, the first thing that may come to mind in terms of roles is an slow but powerful anti-threat. Basically, with the presumed popularity of many threats in addition to Mega Evolutions such as Lucario-M and Gengar-M, Banette can be counted on to keep some of them in check thanks to priority Destiny Bond and Shadow Sneak. It can also temporarily stop defensive teams in their tracks with Taunt and Will-o-Wisp, though its actual effectiveness with both of these moves remains to be seen. Additionally, its Ghost typing obviously allows it to block Rapid Spin, and with its options, it can keep several common Rapid Spin users in check. Of course, Mega Banette is limited in terms of bulk and offensive options, but it has just enough to keep the opponent guessing. Custap + Destiny Bond Wobbuffet, v2.
Possible Moveset(s):
Keep in mind that this is tentative!

name: Prankster (Mega)
move 1: Shadow Sneak
move 2: Thunderbolt / Thunder
move 3: Taunt
move 4: Destiny Bond
item: Banettenite
ability: Insomnia / Frisk
nature: Brave / Naughty
evs: 252 HP / 252 Atk / 4 SpA
This is a basic mixed-offensive/stallbreaker set desgined to keep Mega-Gengar in check, and use its priority Destiny Bond to take down a dangerous threat should you feel overwhelmed (or desire a tie instead of a loss, should circumstances come to that). Shadow Sneak is obviously the main move for the common fast Psychic- and Ghost-types flying around, and has become more useful this generation thanks to Steel's neutrality to it (so it can check Jirachi in the future). Thunderbolt and Thunder are simple coverage moves, and ensure Banette can just put Skarmory out of commission rather than stop it temporarily with Taunt. Speaking of which, Taunt gives Banette a way to play around defensive Pokemon, preventing Ferrothorn and Blissey and other defensive behemoths from trying to set up on it. It's also a way to ensure that the Pokemon attacks Banette, which it can use to lure them into Destiny Bond. Will-o-Wisp can also replace Taunt, though Banette is better off suiciding against Lucario-M and other attackers anyway.
Other possible sets: Offensive set with one support option, and stallbreaker set (which may have competition with Sableye).
Checks and Counters:
As long as Banette is alive and it's able to use its priority Destiny Bond, you can't exactly knock this thing out safely. Like other Prankster-reliant Pokemon, however, Banette's user is also prone to messing up in mind games, so random or well-timed attacks should eventually put it down, though again, beware of Destiny Bond. The more safer approach is to continually force it out and use entry hazards to wear it down until it faints. Tyranitar is a cool counter (answer: Destiny Bond), since it can stop Banette from safely switching out with Pursuit, and wear it down with sandstorm. Banette also can't switch into many attacks due to its average bulk, and easily falls to faster forms of priority if they exist.
Personal Opinion and Conclusion:
Man, just...priority Destiny Bond, man. It's not the greatest Pokemon, Prankster user, nor Mega evolution out there, and it would have rather preferred a Speed boost. However, with the amount of dangerous things out there, I think Banette may actually turn out to be a cool option, especially considering it was sort of needed last generation with threats like Scarf Salamence and Dragon Dance Dragonite running amok. It also has problems facing off against a high number of Pokemon, but, again, the answer too all that is Taunt and Destiny Bond. It's a solid thing, by itself, and a decent addition to teams that find themselves easily overwhelmed by a specific threat, especially considering most people are gaining fetishes for Gengar. I predict it'll find itself a great home in a lower tier, but still have great presence in standard play.