The set I use:
Magearna-Original @ Assault Vest
Ability: Soul-Heart
Tera Type: Ground
EVs: 248 HP / 68 Def / 192 SpD
Calm Nature
- Fleur Cannon
- Ice Beam
- Volt Switch
- Tera Blast
I am of the opinion that Magearna has an actual place in Nat Dex Ubers, a niche place, but a place nothing less. So what does this "place" have to with anything, the AV set is what I'm gonna mainly talk about. The added special defense means that it can sponge special hits quite well, but nothing more important than being an excellent check to the incredibly centralizing Yveltal. Since both of Yveltal STABs are resisted, plus the fact that Magearna can hit Yveltal for Stab super effective damage with Fleur Cannon, which even without investment, will always OHKO Life orb Yveltal after the Orbs damage.
252 SpA Life Orb Dark Aura Yveltal Dark Pulse vs. 248 HP / 192+ SpD Assault Vest Magearna: 52-62 (14.3 - 17%) -- possible 6HKO
0 SpA Magearna Fleur Cannon vs. 0 HP / 0- SpD Yveltal: 398-470 (101.2 - 119.5%) -- guaranteed OHKO
This doesn't just apply to just Yveltal, with it's Zacian-Crowned shared typing, it can also stonewall Deoxys-Attack, thanks to it's expanding Force resist, and solid bulk accompanied by Assault Vest, and partial fairy typing making it neutral to Low Kick, along with easily switching into special moves that aren't Fire or ground type, or Primal Kyogre.
Now Magearna has alot of flaws holding it back from ever being anything above a C+, like a horrible Primal Groudon match, being setup fodder for Zacian Crowned, unimpressive special attack especially without investment, the Ho-oh issue, and lack of reliable recovery.
However, these can somewhat be undone with careful teambuilding, like Alomomola wish passing as an example, so Magearna can last longer in longer battles.
Anyways: https://replay.pokemonshowdown.com/gen9nationaldexubers-2499243966-c6mkc3xh97y5knj2au9l009u0154foupw Magearna's special hit taking abilities at full effect.
https://replay.pokemonshowdown.com/gen9nationaldexubers-2499251650 Not too much, but can prob show it's capabilities against stall quite decently thanks to volt switch.
-> A+
-> B-
As Swas mentioned, I'm really not seeing what Magearna is doing that Fezandipiti doesn't in this sort of role. Magnearna was a Pokémon I loved during the Xerneas metagame, but I've felt it is somehow unviable despite that typing and movepool pretty much since Xerneas was banned. 
Sure? Not really opposed to it being ranked and it does the job fine. 
It is a meme, but also kinda legitmately viable, even if it is only C-. It does fill a niche, and that niche is actually somewhat valuable. The suspect is over and sticky web is staying so having a bunch of shitmons bloat the bottom of the VR that do little more than antilead Smeargle is kinda reflective of the metagame. 
I disagree with this nomination. Tera is a fine price to pay to nullify the antiwebs 'tech' and I don't think Smeargle is an A+ Pokémon regardless of the EV optimisations. The same applies to Ribombee. I think these are cool, but I don't think these are particularly game changing. I do think Smeargle is the best A rank Pokémon, but don't think it is A+. I also don't think Kyurem-B is a good example to use for Ribombee since Tera Electric is the most common and well...with that it gets +2 and has the bulk to deal with Ekiller so it really doesn't matter. I think these two Pokémon are fine where they currently are at the end of the day. 
I get where the nomination is coming from, but disagree. I wouldn't be shocked if Ekiller ends up rising anyways, but I still view it as a near HO exclusive Pokémon and have not loved the other teams I've seen that try to make it work. The competition from Arceus-Ground is a bit too high given the limited scope of teams that Ekiller fits on. It has some A+ vibes to me, but I think it needs more success outside of HO where it is demonstrably an important component of those teams. 
Agreed on this one. I was split between A+ and A for Eternatus last slate and iirc it ended up being a single vote from remaining in A+. Eternatus is sort of the same thing as always: some set fits on a lot of stuff, but I feel that the support it needs in the builder regardless of the set is a bit too high for S- right now. I'd be very surprised if Eternatus didn't rise to A+ this slate and wouldn't be shocked if ended up in S- as I'm likely a tad lower on it than other voters.
Agreed, but also just trapping Zygarde is fine. With Gothitelle imo you're always building a team with the assumption that it will trap some specific thing and it has important enough targets to build around. We are a long way from the days where Gothitelle could semi reliably trap two Pokémon on balance structures though. 
I can kinda see C-, but C is a bit high for me. Zekrom is one of those Pokémon that does stuff, but that stuff rarely feels important enough to justify giving it the extensive amount of support it needs. Even the 'free' matchup of stall isn't nearly as brainless as people imply imo even if it is a good one. Zekrom hasn't seen much successful experimentation in a while, but I think it has some potential to be a low tier shitter. It has just been so long since it has done anything to actualize its potential and I think we are at the point in the metagame where results are needed. 
I'll probably vote B+ this slate, but Mega Salamence gets a lot of undeserved glaze imo. It isn't bad, but it is a rare HO Pokémon that actually gives Zygarde the space to be a threat rather than trade with something which I really don't like. Also...for all the success it has had this year it is really only two teams. Yeah it can be a big threat after a boost, especially against offense, but even getting that is so much more difficult than it should be. It is fine in B+ I guess, but also I think it'd be fine in B. At the beginning of the year Mega Salamence was bordering on C+ imo so it has improved a lot. I wouldn't be shocked to see it in A- by the end of the generation if people 'figure it out' as it feels like there is untapped potential atm. 
A+ -> S- 
A- -> A
B+ -> A-/A
B- -> B
S+ -> S
A -> A-
A- -> B+
B+ -> B
B+ -> B
B -> B-
S- -> S
C -> C+
C -> C+
C -> C+
C -> C+
C -> C+
D -> C-
UR -> C-
UR -> C-
B -> B-
B- -> C+
C -> C-
C- -> D
C- -> UR
There was a fair bit of discussion about Ho-Oh's rank, more than the past couple of slates. Ho-Oh is undeniably one of the best Pokémon in the metagame, but offensive structures utilizing other means of hazard control have been gaining popularity in recent months - namely those that use Pokémon such as Mega Diancie, Choice Scarf Yveltal, and occasionally Mega Salamence. If you're not using these structures you're likely using Ho-Oh. It is still an amazing Pokémon due to its irreplacable blend of offensive and defensive utility alongside providing the best hazard removal the tier has to offer. However, if this trend continues it wouldn't be surprising to see Ho-Oh drop to S- in the future.