S3 has had some interesting developments: it started as what was essentially "season 2.5" due to Zygarde's introduction not really making a splash. Mewtwo joined the fray and immediately warped the format around it. And now, with the DLC coming out in a few days, there's gonna be new moves. Thus, the entire meta will be uppended anyways before S4 shows up, so might as well do a VR:
Nothing is ordered, and the line between what's niche / what's less niche starts getting blurred by B-, and realistically there's C mons that can be there. Idt it matters too much beyond "these are the ones I think you should respect more". C rank is anything that I have used / seen that is workable, and could be a bigger subrank - this game is just that good at making mostly everything viable. Observations:
Xerneas is still the king of the format, and while it has to compete with Mewtwo as both a Restricted Slot and as something that may lose to it, the deer has adapted to Mewtwo's presence with physical sets becoming more common than before, being able to Megahorn the genetic mutant out of the field. And the rarer Mixed GeoXern are still very potent, being able to do their usual shenanigans while employing anti-Mewtwo tech w/ Megahorn and their own increased Special bulk.
The star of the show for S3 is of course Mewtwo. The Megas are so good that they're top-tier calibre, with MMY being the best sniper in this format w/ its blistering speed and beautiful coverage, and MMX being a phenomenal bruiser that can best even its supposed checks (like Xern or Yveltal) while also possessing the ability to thwart screens. Regular Mewtwo can viably run LO and allow for other Mega options, though its slower speed makes it a less attractive option than its Mega brethren.
Chomper is weaker than before, having to contend with Mewtwo's presence by now slotting things like Crunch / Shadow Claw, and being a prime target for Ice Beam. Still does Chomp things.
Mega Gary is my preferred beatstick Mega (even over Mega Gross); matches up well against Mewtwo, and is tanky enough to be able to claim various kills in brawls before it goes down.
Yveltal is something I wouldn't call "worse" than it was before per se, but there's just more opportunity cost for the Restricted Slot now w/ Mewtwo's introduction, and its "jack-of-all-trades" status isn't as lucrative. It can potentially beat Mewtwo as well, though doesn't have the best matchup against either of the Megas so has to thread carefully.
I probably rate Zard X higher than most - I think well-played ones run away with games, and base Zard being absolutely horrendous is salvaged by its ability to dodge Ground moves and bait Thunderbolts.
I acknowledge Exca as a real mon only with Focus Sash; I rate it higher this season because it can choose to check Xern or Mewtwo depending on your team's needs, and Ground STAB is still obviously great.
Clef is Xern-lite, mostly fitting into comps where you want to use another Restricted slot (like Mewtwo). With its fast movement speed, offensive Clef can pick up kills with its beautiful coverage, with Ice Beam being a notable boon over the deer, and dual-screens Clef can still do its thing.
Mega Metagross is overrated, and the more time passes, the more I question its insane usage. That said, its stats and ability to reliably check Xern are what let it get away with what it gets away with.
The ABSOLUTION. I ran this all the way back in S2 since I like finding niches in mons, and I was very impressed with its ability to deliver kills. With the release of Mewtwo, it's shining this season, and
Vengeance417 has already gushed about it above haha. It's not the easiest mon to use due to its frailty, and the fact that you cannot chase with it, but overcoming that ceiling is pretty rewarding!
Tyranitar is a real mon now that Mewtwo is here.
Zygarde-10 has fast movement speed and can launch Land's Wrath / Thousand Arrows off a decent Atk stat, and its short stature also allows it to dodge funny stuff at times. Zygarde-50 is way too slow, and Complete is clunky. The Mega is too exploitable in this format, and the funny "THING GO BOOM" move does not salvage it.
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Anyways yea that's kinda it. The DLC is gonna heavily change the remainder of the S3 metagame with its new moves, and I wouldn't be surprised if we see random things like Gallade go to A+ rank or some shit lol