I agree with mega altaria in B-, (speaking for the DD set, i dont have enough experience with the special set for me to say much about it) although I honestly wouldn't mind if it dropped to C+. This was one of my favorite mons to use back in ORAS, so I obviously rushed to test it after it was released. I believe it does have a niche in OU for its ability to set up in the face of very prevalent mons and threaten them like koko, tangrowth (yes they both run hp ice but 0 SpA Tangrowth Hidden Power Ice vs. 0 HP / 0 SpD Altaria-Mega: 90-108 (30.9 - 37.1%) -- 72.9% chance to 3HKO and 252 SpA Tapu Koko Hidden Power Ice vs. 0 HP / 0 SpD Altaria-Mega: 102-122 (35 - 41.9%) -- guaranteed 3HKO) and zygarde w/o iron tail, zapdos (has to be careful of paras if its not the refresh set), and latio(a)s. Its bulk and amazing typing also allows it to sponge hits from common, relevant mons like ash gren, keldeo if necessary and can threaten out many more like tyranitar (mega or not), bisharp, heracross-mega and lopunny with a STAB pixlate boosted return/double edge (i personally don't like double edge. the fact that it is pressured constantly throughout the game means that it doesn't appreciate recoil from double edge
at all). it can also threaten the duggy/zard y core running around as it can act as an emergency zard y check and also has the merit of not prone to being revenge killed by duggy.
that being said, why am i not advocating this thing to rise, if it has so many positives? Well for one, its speed tier is atrocious, even after a boost it fails to outspeed important mons like scarf lele and scarf protean gren (and to a lesser extent, nheligo, but i havent seen that thing in a while), (i dont think outspeeding koko or ash is too relevant tbh cuz it can boost on them...) and is usually forced out by these mons. In addition, the presence of celesteela, scizor and skarmory stop the DD set cold, as it can hardly afford to run fire blast and unfort altaria teams usually need to run zone to eliminate these mons- if skarm/celesteela are alive, altaria usually sits there in the back and doesn't do anything. finally, the pixilate nerf also sucks, because now it can't 2HKO unaware clef like it used to.
last gen:
252 Atk Pixilate Altaria-Mega Return vs. 252 HP / 172 Def Clefable: 204-240 (51.7 - 60.9%) -- 94.1% chance to 2HKO after Leftovers recovery
(that was the clef set that was relevant last gen)
This gen:
252 Atk Pixilate Altaria-Mega Return vs. 252 HP / 172+ Def Clefable: 169-201 (42.8 - 51%) -- guaranteed 3HKO after Leftovers recovery
TLDR:
- Altaria has a strong niche in the OU tier for it to be ranked bc it can set up in front of some common pokemon and threaten things like veil (sets up in front of zygarde and threatens it and kills the other mons as it resists sucker from mawile)
- its typing also allows it to threaten and sponge hits from some other common pokemon like ash gren, keldeo, latios etc.
- the prevalence of celesteela and scizor stops the DD set cold
- its speed is bad this gen and some faster scarfers able to revenge kill it even at +1
- i advocate its ranking in B- or lower to C+, although im not personally nominating it to be lower.
just one last quick opinion:
Keldeo ==> A-: agree. the rising usage of fat pex/steela/clef/venu cores doesn't help this mon at all, and the fact that mantine has been seeing some usage is kinda bad for it too (hp electric means u drop stoneedge for volc), and i believe it should drop.
for the other noms i dont have anything much to say, i dont feel strongly about any of them.