I am here to advocate on the behalf of a mon very near and dear to my heart which I believe deserves a good solid look in the current meta, who has improved drastically in usability now that Horoark is dead. LBN views this mon as simply not the best but
I see potential... and a very valuable niche, an anti-meta pick in a way.
The famed Bellibolt of RU fame has fallen outta use in recent years, but unlike other mons of its tier and calibur it's still quite good, and packs some solid defensive benefits. First off is that defensively, Bellibolt just has solid stats. 109 HP, 91 Def, and 83 SpD help it live a boatload of hits, and paired with Slack Off makes defeating it a daunting task for wallbreakers to take down. But I'll cut to the chase and get into the niche that I believe Bellibolt fits into in the current metagame.
I believe that Bellibolt is the single best Pokemon on fat for taking on Offense teams. Specifically Gapdos Volc Mamo offense. Hyper Offense it's fine into but that's irrelevant.
I'll also preface that you shouldn't be running volt switch for this purpose, run Parabolic Charge so you can heal up and stay topped off into fools or Discharge so you can fish for paras and do better into special attackers or guys trying to set up on ya. I personally prefer Parabolic Charge cause it lets you not only stay healthy vs. Gapdos, but more importantly lets you stay healthy vs. Taunt Volcanion due to having healing that's not tied to a status move.
Additionally, you have flexibility with the ability, Static lets you better punish regular Gapdos sets w/ scarf, and if you're worried about Protective Pads on gapdos, then you can go for Electromorphosis, which lets Parabolic Charge hit way harder, and lets you answer back easily. Otherwise you can run Static to better punish other various contact gamers like Mienshao, Bisharp, Maushold, etc etc. It also lets you become the world's best Jirachi switch in, chipping w/ rocky helmet AND threatening a Static para.
Overall comes down to how scared you are of pads and if you worry about how well the rest of your team can follow up on the para. TLDR, Electromorphosis helps Bellibolt be self-sufficient, Static helps it be a team player.
But otherwise you may wonder how you beat Volc and Mamo? It's honestly just as easy as Tera Water and win.
I know I know! The thought of "burning tera" is a terrifying one, especially in the RU metagame and for good reason. You want an out, you sometimes want offensive power, you want to be able to eek out a free turn in the endgame. But Bellibolt being able to simply take on 3 mons at once for multiple turns at a time is like amazing value, and it's not like it opens up a major weakness or anything, you just become a bulky water.
And, more importantly, you can pull the trigger at any time during the match. Need to eek out a win vs. Gapdos Mamo endgame? Bellibolt wins. Need to lessen the strain on your defensive walls? Take out their Mamo and get to work. Additionally, Tera Water Muddy Water from Bellibolt does 83% minimum so some chip lets you kill it easily, AKA SR chip if they're CB.
0 SpA Tera Water Bellibolt Muddy Water vs. 0 HP / 4 SpD Mamoswine: 300-354 (83.1 - 98%) -- 68.8% chance to OHKO after Stealth Rock
So your Mamo MU is fine if you Tera, baring ungodly bad luck from Icicle Crash flinches. And, suprisingly, your Volc MU is also solved!
252+ SpA Volcanion Earth Power vs. 252 HP / 0 SpD Tera Water Bellibolt: 126-149 (29.8 - 35.3%) -- 23.6% chance to 3HKO
Just look at this.
Scariest wallbreaker in the tier and 76.4% of the time you get 4HKO'd.
However, what sets Bellibolt from other bulky waters that put up more impressive results is that
Bellibolt can actually answer back with something. Milotic, Vapo, Suicune, Empoleon, all scratch and slightly annoy Volc, and all kinda get shut down either by Earth Power or Taunt or by merit of just wasting PP. None of them can actually
directly damage and threaten Volcanion, unlike Bellibolt who has it's Electric STAB of choice OR Toxic if you know they're not Taunt and want to midground in case they switch to a ground.
And of course you have a defensive piece that at least gets rocky helmet chip into Gapdos, and at most either murders it with Electromorphosis Parabolic Charge or gets a Static Para. But you know what Bellibolt in base form also does?
RESIST BRAVE BIRD
Even when Tera'd you don't take like an insane amount from Gapdos, it's enough to raise an eyebrow, but it's not crazy damage, and you can usually Slack Off or Parabolic Charge. Latter if they're Protective Pads.
252 Atk Zapdos-Galar Close Combat vs. 252 HP / 236+ Def Tera Water Bellibolt: 148-175 (35 - 41.4%) -- guaranteed 3HKO
So after I've put this all out, you can surely see what I mean about Bellibolt right? This is a physical wall that can handle Gapdos, Kill Mamoswine, and NOT DIE TO VOLCANION AND LIKE ACTUALLY BEAT IT. You have healing that's not tied to a status move, an actual status move that heals you, Toxic so you can screw over bulkier mons, and if you really need it you can gamble on Muddy Water and it's 30% chance to potentially let you win a last mon situtation.
And this isn't like doing any assumptions, these aren't cherry picked situations or anything, like Bellibolt can just do all of this.
Obviously though, there are more than 3 mons on these Offense teams. But that's where every other Pokemon on YOUR team comes in. If they have a Bisharp, then you should probably have answers for that, like the big idea here is that you don't want to have Bellibolt handle absolutely EVERYTHING on the opposing team. Bellibolt can handle the big classic brainless offensive 3, but it'll probably perish in the act, so being able to handle problem matchups, handle Jirachi clicking body slam, handle setup sweepers like Oricorio clicking taunt and QD, that's up to the rest of your team. But Bellibolt, good ole Bellibolt, is perfectly capable of taking on the 3 goobers for the rest of your team in a way that role compresses BIG TIME.
So give Bellibolt a shot! It's a very versatile mon that honestly can fit onto more than just fat, and is capable of adapting to what your team's specific needs, so if you want an answer to Gapdos, Mamoswine, and Volcanion in one silly little tera water, Bellibolt is your guy!
Have a great day y'all!
Late addendum but please ban oricorio. 
Okay, we actually got something, because if we didn't, I wouldn't be making this post. Mew is a classic case of jack of all trades, master of none. Main sets are going to be NP mew, spike stack mew and hazard lead mew. None of these sets will be amazing, but another option in the tier will be very nice.
#youtuber_gets_bad_mon_to_rise. Okay, feraligatr isn't that horrible, but it rising to RU isn't on my bingo card. Main issue with it is that gyarados is just better. Moxie allows gyarados to snowball better, its defensive utility is better with its typing and good special defense, and while feraligatr has higher power, power isn't everything.
Speaking of gyarados, yeah, this is a good ass mon. Resto chesto can setup on a lot of defensive pokes really easily, while 3A is still deadly. Just a good allround HO mon. 
Pom pom is one of those 'bitch mons', where its not broken, but oh fucking boy, it will make you tear your hair out facing it. It has two sets, the dual stab set which uses tera ground or fighting to make revelation dance hit mons such as goodra and registeel, while taunt sets made defensive teams living hell.
Feli would be proud rn. Registeel has shown itself to be one of the best defensive steels in the tier, with ID+BP being an insane wincon while still being able to get up rocks or paralysis something with thunder wave. 
