I already covered Manectric in one of your previous questions, but to those who haven't seen it:
Manectric is a common Pokemon obtainable fairly early at Route 10 and probably one of the better obtainable Electric types in the game. It's fast, it has a good Special Attack stat, a good Mega Evolution, and a decent movepool, though through level-up it's fairly shallow outside of Electric-type moves.
Its overall good stats and solid matchups against key gym battles (especially Winona and Wallace) means that Manectric is a very hard Pokemon to pass up. I greatly recommend it.
One of the best Electric types obtainable
Learns Discharge fairly early (Lv. 30 in ORAS)
Access to a Mega Evolution
Fast with good stats (105 Sp. Atk and Speed)
One of the best users of Overheat
Limited level up movepool outside of Electric type moves
Now onto Banette. It's... an interesting choice, to say the least. It boasts a surprisingly high Attack stat and a very solid Special Attack stat, but its other stats aren't great, especially Speed. You can also nab its Mega Stone very easily in Mt. Pyre. Unfortunately, the earliest you can catch a Shuppet is all the way in Route 121, and by then, quite a lot of people have already solidified their teams by then. However I don't think it would be quite fair to immediately dismiss Banette as "too late to be useful" as it's actually quite useful against the upcoming gym battle with Tate and Liza, by virtue of its Ghost type. Its movepool is actually pretty great, at a first glance. Will-o-Wisp? Knock Off? Shadow Claw? A bunch of great TMs? Sadly, once it gets Sucker Punch, it learns nothing useful through level up. If you hold off on catching Shuppet at a later level, you might need a Heart Scale to reteach it Will-o-Wisp if you don't have the TM, and you definitely need one for Knock Off if you want it. With its usable Special Attack stat, it can effectively use Shadow Ball and a wide myriad of varied TMs if you choose.
In general, it's a little hard to recommend, but considering you asked about Mightyena as well, it'd be a perfect time to drop it for Banette if you don't want to use the Box Legendary.
Ridiculously high attack stat with a decent special attack stat
Access to a good Mega Evolution
Obtained fairly late in the game
Lacking stats when not Mega Evolved
Pretty slow (65, 75 when mega evolved)
Not enough useful moves to utilize Prankster with in the main story (Destiny Bond is an egg move)
Who do I recommend? Manectric is still amazing, but Banette makes a good replacement for Mightyena or a last second final party slot.