So lets discuss some actually viable Pokemon because right now there's like nothing actually ranked and I didn't even see any Vivillon used by crappy players let alone anyone decent when I was at the Mass regional.
Aegislash: S-rank. A ton of people are using Aegislash right now, and for good reason: it's bulky as all hell but still has a ton of power behind it. It checks Mega Kangaskhan, Mega Pinsir, Mega Mawile, Mega Venusaur, Mega Charizard X, Amoonguss, etc etc etc. King's Shield is such a good move that a lot of Smeargle even forgo Protect for it, since the -2 Attack is key to fending off things like Mega Tyranitar / Mega Kanga w/Crunch / other stuff. The standard set of Substitute / Flash Cannon / Shadow Ball / King's Shield can totally hold its own in the metagame, since there aren't many Pokemon that resist both moves. If standard sets aren't your thing, it gets some slightly more niche options that can support your team if you need it. Wide Guard / Destiny Bond are notable egg moves, Wide Guard in particular being useful to get around Earthquakes / Heat Waves. HP Ice is a niche option but it can surprise Garchomp and Salamence because Aegislash can easily tank one Earthquake or Flamethrower respectively. Physical Aegislash is a little less useful than special but Shadow Sneak is a cool option to pick off some weakened Pokemon, and Sacred Sword absolutely wrecks Tyranitar. All in all, it's a great Pokemon that has a ton of utility on many types of teams. I could continue to go on with all its options, but I think I made my point.
Tyranitar: S-rank. Great power, great bulk, decent enough typing that gets the job done, covers weaknesses nicely with cover options. This thing is super versatile: you can use anything from a special attacking set with Choice Scarf to a somewhat more bulky attacking set to an almost defensively bulky set that still has enough power to wreck frail Pokemon. This isn't even considering Mega Tyranitar, which is a whole different animal (fun fact: 0 hp / 4 def Mega Tyranitar survived an Escavalier Megahorn with 12 hp left and then still had enough bulk to take a Mega Manectric Snarl). There are very few teams that get worse by adding a Tyranitar, unless it's like a dedicated Rain team.