Alright so imma drop my unprofessional two cents on the OU/UU changes
Mew moved from UU to OU : Eh, I don't use Mew often unless I need a fairly bulky defogger.
Beedrill-Mega moved from OU to UU: Oh come back to me, glorious adaptability U-turn! Seriously, this was all I was using back in Generation 6, and hopefully it's still as glassy as it was, then and now.
Sceptile-Mega moved from OU to UU: Yippee. I think I tried making a Baton Pass set with this before the ckause. Either way, this one seems fairly fine. At least Dragon types have another thing to fear now.
Mamoswine moved from OU to UU: I'm more hyped for this one, as I use Mamoswine a lot. Mostly for the Endeavor + Ice Shard suicide leads and the resistance vs Ice. Plus, Stealth rockers! There wasn't enough of them.
Muk-Alola moved from OU to UU: Yet again another powerful Pokemon slips in tier. Considering how Poke like Audino-Mega are moving to UU, this makes sense. Maybe an Assault Vest Anti-Xurkitree set? Ideas.
Clefable moved from UU to OU: The metagame ( in my eyes) lost a vital Pokemon now. Or at least, a very potent anti-sweeping machine. The fact that the likes of Feraligatr, Tail Glow Manaphy, Z-Hypnosis Xurkitree are all very real now solidifies the impact of this change.
Salamence moved from OU to UU BL: Back where you belong. Anyways, it mostly gets outshadowed by the more potent Darmanitan, although Outrage spam is still going to be very scary for many teams.
Kyurem-Black moved from BL to OU: Subzero Slammer is more popular than I thought, imo.
Steelix-Mega moved from OU to UU: Maybe as a very bulky defensive wall and potent anti-fairy counter, it still doesn't like special moves. Either way, I am excited to start doing UU again with this thing on teams.
Buzzwole moved from OU to UU: IMO, the threats of death from Flying Pokemon, not to mention Psychic and fire, must have made Buzzwole drop. Even as it is, I can see it dropping to lower tiers; Buzzwole's weaknesses are mostly special attackers, while flying does four times damage.
Pidgeot-Mega moved from OU to UU: Eh. Aside from a threat of Hurricane/Heat Wave spam, Pidgeot-Mega did not really stand out to me in the meta-game. I'll probably give it another shot now, though.
Crawdaunt moved from BL2 to UU: Yikes. I've been asleep too long; when did Crawdaunt become a threat? I always found Azumarill to be the superior Water-type setup sweeper, so unless it's for speed, not seeing why Crawdaunt should be getting used more, especially since Fairy types hit super effectively against a Pokemon with 63/55 special bulk.
Houndoom-Mega moved from OU to UU: Houndoom-Mega started a huge debate on whether the usage of Sun was a problem or if sun-casters were the problem or if Houndoom-Mega was the problem. Honestly, I didn't hate it as much as others did: It needed more team support than I'd like to admit and, to be honest, was more frail than anything.
Smeargle moved from NU to OU: Surprised it's not been in OU for this long. Smeargle is what made BPass broken and now it's wrecking havoc elsewhere, spreading sticky webs and spores for all that crosses it.
Keldeo moved from UU to OU: No more scarf Icy Wind/Hydro Pump to deal with? I always found Keldeo an annoying Pokemon to deal with, mostly because a huge part of my teams in general hate the typing Ice.
Bisharp moved from UU to OU: Nice, Glad to see it advance, although I never did see it as much of a threat Mach Punch/Vacuum Wave fodder but hopefully in OU Bisharp will find it's niche again in...Sucker Punch spam? At least it's Anti-Psychic.
Pelipper moved from OU to UU: It's funny how at the same time Pelipper comes out, Xurkitree does-but then so does Mega-Swampert and Crawdaunt. At least there's a potent, viable soft counter to Houndoom-Mega now rough day for him Anyways, Pelipper will mostly be the rain and defogger that it was in OU, except with the new Pokemon included, I can't see it dropping past, say, BL2.
Xurkitree moved from OU to UU: God help us all, Z-Hypnosis and Tail Glow sets are going to be everywhere. With the addition of Pelipper-which, against it just provides Thunder spam and setup fodder, along with Dazzling Gleaming or Tbolting a lot of Pokemon currently and introduced in the meta, Xurkitree will be a force to be reckoned with.
Nidoqueen moved from NU to UU: Oh, that's neat. Never got behind the sudden Nidoqueen love: maybe there's a really good competitive niche to it?
Audino-Mega moved from OU to UU: Eh. Don't know what to say about this one.
Gengar moved from UU to OU: This goes to show that even without Levitate, Gengar is still a very good 'mon Pokemon to be using. No more Scarfball temporarily-or at least damn near un-outspeedable ones.
Swampert-Mega moved from OU to UU: With the introduction of Pelipper , M-Swampert looks like it's going to be a very good soft Thunderbolt Check against Xurkitree. Unless it's Sashed, Earthquake is going to kill.
'Scuse me, I need to make some Tail Glow spam teams.