I'll give this a shot. My thoughts on the current slate and some other suggestions people have made:
Empoleon:
B- to B
I'm not so sure on this one, but I'm leaning toward agreeing. As others have said, Empoleon does a unique typing and good movepool (albeit without reliable recovery), but it has trouble functioning as a fairy check. Air Balloon/Chople Berry sets miss Leftovers recovery, and Leftovers sets are weak to common ground/fighting coverage moves from fairies and other mons. Still, I think the fact that it has so many options at its disposal like Scald, Defog, Roar, SR, Ice Beam, Flash Cannon, etc. puts it into B for me.
Skarmory:
B+ to A-
I definitely agree with this one. Between its specially defensive and Custap Berry sets I think skarm deserves a rise. Reliable recovery, Taunt, SR, Spikes, BB, Counter, Iron Head, Defog, and a decent ability in Sturdy - Skarmory just has a ton of options it can utilize effectively. Its suicide lead is incredibly effective on HO, and being the only mon who can reliably get up two layers of entry hazards for these teams warrants A- imo.
Mega Medicham:
C+ to B-/B
I agree with this totally. At the start of ORAS, Gallade's superiority to M-Cham was emphasized heavily, but as the metagame has adapted and changed, it's become clear that each of these mons has their own benefits. Cham has distinguished himself from Gallade because he has more immediate power and double priority, while still having decent speed. Gallade still has his advantages, but the gap isn't as big as it was at the start of ORAS. M-Gallade was already moved down to A-, and I think M-Medicham moving up would more accurately reflect the difference in viability between the two.
Starmie:
A to A+
I actually disagree on this one. While Starmie's defensive spinner sets are powerful in the current metagame, they have to chose between beating pursuit trappers and ferrothorn or beating Keldeo, who we all know to be an incredibly potent threat right now. Offensive spinner sets are good too with amazing coverage options, but I find that they ultimately lack some power due to Starmie's mediocre base 100 special attack. I'll admit that Analytic can remedy this somewhat, but that 30% boost rarely activates outside of the opponent switching, and a lot of the time Starmie wants to rapid spin on the switch anyway.
Tyrantrum:
C+ to B-
I haven't used Tyrantrum at all, but from what I've seen its banded set alone warrants a rise. Banded STAB 150 BP move with no drawbacks? Yeah, this thing is a nuke.
Alakazam:
C to C+
I agree with this one. Focus Sash/Twave sets are a nice emergency button for the plethora of set-up sweepers in OU, and Alakazam's fantastic speed tier and good Special Attack lets it act as a nice revenge killer too. Life Orb sets are pithy with coverage options, and again Zam's great speed tier lets it perform very effectively. I don't think Zam should go much higher than C+ though because its pathetic bulk can only get it so far.
Forretress:
Unranked to D
Despite its unique ability to set up hazards and rapid spin hazards on its own side away, I just have a hard time seeing Forre's niche in OU. It's far too passive and susceptible to set up sweepers. I vaguely recall reading some talk about dividing the D ranks into subcategories. If that were the case, I think I could see Forretress as a D- rank. I know the distinction may seem silly/small, but hey, my opinion is my opinion.
Hydreigon:
B to B+
I agree with this. Hydreigon with a LO is just such a powerful threat given its incredibly diverse movepool, and everything that isn't a fairy or a pink blob fears its STABs.
Mega Alakazam:
A- to A
Mega Alakazam is absolutely my favorite mega pokemon to use in OU, but I don't think it deserves to rise at this point. I think that MegaZam is definitely a threat that is underprepared for, but its susceptibility to priority really inhibits it when pretty much every team outside of maybe stall, I guess, carries a priority user. It's speed tier and base special attack and neutral coverage options are truly amazing for a late game cleaner, but its reliance on the shaky accuracy of focus blast is problematic as well. I think A- is a good place for MegaZam.
Slowking:
B- to B
Someone brought this up a few pages ago, and it didn't get much discussion, but I wanted to emphatically give my support for this nom. Slowking is great at checking a plethora of common OU mons. With an Assault Vest and certain coverage moves, it can handle non-Pursuit Mega Metagross, non-Knock Off Landorus-I, Keldeo, specially based Mega Altaria, Gengar, Heatran, both Latis, Char-Y, Ferrothorn, Gardevoir, and non-Energy Ball Manaphy from the S through A ranks. Slowking also has a fantastic movepool with good stabs and great coverage options like Ice Beam, Fire Blast, Power Gem, and Dragon Tail for phazing. With some defensive investment, Slowking can even beat some pursuit trappers like ScarfTar. Its susceptibility to the ever present Knock-Off and the physically oriented metagame are definitely drawbacks, but I still think Slowking deserves a B rank.