I'm going to go ahead and say that Garchomp should neither rise nor drop. It's perfectly fine at A+, and defines that rank pretty well. It has a multitude of usable sets, is effective in many scenarios, and only needs minimal support to function properly. It has some exploitable weaknesses, but can usually find a place in battle every time, and is something that every team should have an answer for.
Mega Banette might have enough going for it to be D rank... but I'm not completely sure. The main reason that it's unranked even though it does have a lot of interesting toys to play with is the fact that there's just so much that it has to compete with, and the opportunity cost is just way too damn high. Even if you could use mega banette to throw around prankster status, it doesn't always mean you should, as there are other pokes that do the same job. Thundy and Sabelye are two that were just mentioned above, and they aren't even the only ones, just the only ones that are OU viable. And when you add in the fact that it uses your mega slot, you had better have a VERY compelling reason why the slot went to banette and not something else. Usually banette's job can be filled by a non-mega pokemon, leaving your mega slot open for something else. But if we WERE to look past all the other options there are and what else could be used instead, banette is actually a pretty decent 'mon. Prankster status is always a plus, able to really punish opposing teams, and prankster Destiny Bond is just evil. It's 165 attack stat is money, and it has a few decent moves to take advantage of it, including STAB priority in shadow sneak. It really sucks that prankster boosts don't have an effect on the turn you mega evolve, so you have to be careful when using it, as you'll have to wait a turn (usually) before you can go around spreading prankster status and what have you, and that hurts banette's case a lot. Really, banette isn't actually that bad, it's just hard to justify putting it on your team, as there are so many other options available. So what you'll have to do if you want to see banette get ranked is find a very good reason to use it instead of everything else that could go in that teamslot, because otherwise, we really can't rank it.