Since this thread hasn't gotten a response in reasonable amount of time, I thought I might as well brake the silence.
A few nominations, not necassarily applying to the lower ranks, but I'm bored so I busted wanted to make a few suggestions:

up to A
Ice is an excellent offensive type in the metagame, due to the presence of the Landorus forms, Garchomp and the ever so annoying Gliscor. Now, both these pokemon play a little differently, the only reason I clumped them together is because I don't have as much time as usual so I just dumped these two together. Anyway, both of these threats are extremely anti-meta right now, Mamoswine getting access to insane stab coverage and power, and Kyurem being the practically uncouterable, and both being the literal poksonification (hehehe adding words to the dictionary) of the devil to switch into. Mamoswine also has access to ice shard, allowing it to easily revenge kill faster threats such as the eon twins, thunderus, tornadus-T, landorus, mega altaria and serperior after little prior damage (or none in Landorus's case). Mamoswine can also smack would-counters with freeze dry, which does considerable amounts of damage to bulky waters and rotom-wash, one of mamoswines best answers. It can even run smash down to hit skarmory (I've tried this, its surprisingly effective and still deals with rotom-wash). Kyreum, on the other hand, is an absolute nightmare for slower teams to face, due to extensive coverage. It's extensive movepool allow it to deal with many hard checks, such as earth power for rotom, hidden power fire for scizor/ferrothorn and iron head for clefable and other fairy fiends, and its 95 base speed allows it to get the jump on mega altaria, landorus-T and mega metagross before mega evolving. Overall, both of these pokemon can be very effective against many teams and deserve a promotion.
up to B+
One of the best leads on offensive teams due to a combination of Speed Boost+Hazards+Baton Pass. Can run moves such as rock slide for Talonflame and iron defence if you want an easier time setting up. Can also lay spikes and toxic spikes to apply pressure. Its late game cleaner set is actually pretty terifying if you loose answers, and can rip through unprepared teams or baton pass to get your teammates to tear them up for you. Somewhat underwhelming nomination, but its pretty straight forward. Forgive me :(.
Up to B-
I know this has probably been bought up multiple times, but I thought I might as well state the case. Yes, you could run tornadus-T and get a similar effective, in addition to a wider movepool and the ability to run another mega, but I what I will be focusing this nomination on is stall breaking potential, A.K.A the work up + plus refresh set. This set is terrifying to face with stall and balance teams, as it can easily blow through pokemon with 100% accuracy hurricanes. Its offensive set can also be a pain to switch into, and counters such as heatran and tyranitar can be dealt with by u-turning into a teammate. Flying, being an excellent offensive type, backed up a 120 base power 0 drawback move is extremely difficult to switch into, rounded up by heat wave, which can do considerable damage to jirachi and metagross, as well as KO scizor and Ferrothorn. The work up set has very few answers on stall teams, and can blow through pokemon such as clefable, sableye, slowbro and tentacruel after a boost. Overall, Mega pidgeot, while having a large opportunity cost is an absolute monster when used correctly, and while it may not be much over tornadus, 100% hurricanes and the ability to wreck stall without relying on taunt is enough to warrant a well deserved promotion to B-.
Currently preparing myself all the hate I'm going to get for this
Other things I agree with:
Gastrodon rising to B-/B (B+ feels like a little much).
Alomamola dropping to C+ (C is a little much)
Gourmets Rising to C+ (B- feels like a little much).
Note: If this post is was not posted at the right time, let me know and I will terminate it.