I realize this is a bit early, but I've been typing at this for a long time and don't want to lose it all, so here goes. Expect this to get updated later this week.
EDIT: Mega Gardevoir added.
Zapdos (B) -> B | Yeah, I think it's about time
Zapdos falls to B Rank. Thunderburd had a niche in being able to check Aegislash and a few other Steels, but with Aegislash banned, a Pokémon that Zapdos could check quite well, it's lost another one of its uses. As Albacore pointed out, Starmie now performs vastly better with Aegi out of the tier and makes for a solid hazard remover. Despite its use as a Defogger, Zapdos suffers from having a SR weakness, which hampers it a lot in that regard. As an offensive Pokémon, Raikou and Thundurus-I do the job far better and Mega Manectric provides useful Intimidate support while having the same Fire-type coverage; it's also much stronger than defensive/Defog Zapdos and has nearly unoutspeedable Volt Switches. Zapdos has good Speed at best, at 100, making it vulnerable to being RK'd as an attacker. Unlike Thundurus-I or Raikou, it has no Special Attack-boosting options and unlike Mega Manectric, running Defog gives it 4MSS: without Heat Wave it loses to several Steel-types and without HP Ice Garchomp and Mega Charizard X shit all over it. Mega Charizard X can just set up on Zapdos all it wants, too.
All in all, I feel like Zapdos has really lost its luster in OU as of now.
Move it down to B.
Raikou (B) -> B+ | I feel like Raikou has become better with the departure of Aegislash. It's a great CM attacker and has just the right coverage to plow through some major OU threats, notably Keldeo, Azumarill, Mega Charizard Y and Talonflame, Thunderbolt doing a shitton of damage to all of them (aside from AV Azumarill). Its nifty coverage move of Extrasensory makes it so that Mega Venusaur doesn't wall it to Hell and back, unlike what is the case with Mega Manectric and Zapdos. Lastly, in terms of moves, HP Ice hits pesky Dragon-types like Garchomp and Dragonite. As for other sets, Assault Vest can check a large myriad of special attackers and Raikou is one of the fastest Choice Specs users in the tier as well, giving it a definite offensive presence in OU.
Now, what holds it back? For one, a susceptibility to being RK'd. Scarf Garchomp just EQs it into oblivion, while Azumarill's Aqua Jet can seriously dent it to name a priority move. It's also set-up bait for some of the most dangerous attackers in the tier, namely Mega Tyranitar and Mega Charizard X, the former of which can use its astronomical bulk to even stomach +1 Thunderbolts and proceed to OHKO with Earthquake, the latter of which can set up a DD on any of its moves and either DClaw or Flare Blitz the shit out of it. It also lacks the precious Fire-type coverage that Mega Manectric has, as a last note. Nevertheless, Raikou is more solid than ever and
deserves to rise to B+.
Mega Heracross (B+) -> A-/A | Where to begin with this monster bug? I'm not kidding when naming this thing monster bug: Mega Heracross is an absolute demolisher. Let's start with its
otherworldly base 185 Attack, giving its strong STABs some real force. Its STABs are none other than good ol' Close Combat and, more notably, Skill Link Pin Missile, which not only outdamages Megahorn, but is also more accurate AND shits all over Substitutes. The only coverage move this Hercules beetle needs is Rock Blast, amplified by Skill Link as well, to have a move that's 1.25x stronger than Stone Edge, 10% more accurate and with twice as much PP. You think that was all there is to it? Fuck no, this thing is so monstrous that it gets Swords Dance to make its Attack so high that almost everything in the tier is OHKOed by any of its moves. With its 80/115/105 defenses, as well as key resistances to Grass, Fighting, Ground and especially Dark, Mega Heracross has ample of switch-in opportunities and with that galactic Attack stat, it forces many switches, giving it many chances to set up a Swords Dance and leave behind a trail of wreckage. Considering its stats and moves, Mega Heracross is one of the ultimate nightmares to and completely annihilates Stall teams.
These amazing traits aside, Mega Heracross has several flaws. Firstly, its weakness to Fairy-, Fire- and Flying-type moves limits its switch-in chances and leaves it weak to several top-tier threats if it can't outspeed these. Secondly, its base Speed of 75 does hold it back from being a total demolisher, since that leaves it quite prone to Offense teams. Fortunately, this can be fixed by using Sticky Web. All in all, Mega Heracross is an absolutely marvelous wallbreaker that's capable of easily destroying whole teams given the chance and should always be prepared for.
Mega Heracross for A-/A.
Mega Gardevoir (A-) -> A/A+ | You thought Mega Heracross was a nightmare to Stall? If that's what you thought, then Mega Gardevoir would be the poltergeist haunting it! Seriously, this Pokémon has become even more amazing than it already was. 165 Special Attack is absolutely fantastic and Mega Gardevoir has just the right movepool to maliciously abuse the living fuck out of it. Its signature Pixilate Hyper Voice has 175 power counting STAB and Pixilate, but its Fairy-type and ability to bypass Substitutes is just mind-blowing. This move alone makes Mega Gardevoir ridiculously hard to switch in on, but this dancer has access to STAB Psyshock to hit physically weak Pokémon and provide additional coverage. The third main move on Mega Gardevoir is Focus Blast, which, while inaccurate, allows it to slay most of the Steel-types that tend to annoy it, including Heatran, Skarmory and Mega Mawile. Since Aegislash has left, these three moves provide perfect coverage in OU and leave room for another useful move to add: Calm Mind allows Gardevoir to raise its Special Attack to insane levels, while giving it notable Special Defense to take hits on the special spectrum; Taunt gives Mega Gardevoir the ability to completely lay waste to Stall teams; and Will-O-Wisp lets it cripple physical attackers for a teammate to set up on or catch said attackers by surprise on the switch to live a physical hit. Additionally, stat-wise, Mega Gardevoir has a wonderful Special Defense stat of 135, giving it notable special bulk that can be augmented by Calm Mind, as mentioned before. 100 Speed is a good Speed tier in OU and since several other base 100s often don't go max Speed, Mega Gardevoir can easily run a Modest nature to make its power simply absurd.
With all these fantastic advantages, Mega Gardevoir does have some notable flaws. Firstly, its measly base 65 Defense gives it no physical bulk if not using Will-O-Wisp and leaves it prone to being revenge killed; Talonflame is the worst offender, as it's immune to burn courtesy of its own Fire-type. Additionally, 100 Speed is good, but not fantastic and leaves Mega Gardevoir outsped by key OU threats, such as the Eons, Thundurus-I and Landorus-I. This issue can be fixed by using Sticky Web, however, and Mega Gardevoir is easily one of its best abusers in the OU tier. To summarize, Mega Gardevoir is a massive threat due to its general unpredictability, jaw-breaking special power and being extremely difficult to directly switch in on.
Mega Gardevoir for A/A+.