Ranking updates. I'm mostly taking what has been seen (so far) from this SPL and in smogtour playoffs into consideration for my changes. Notes applied for relevant drops and rises if I have enough to say. After SPL I will probably just edit this post if I feel it necessary.
S RANK
S rank
Magearna
Landorus-Therian (Gliscor up = Lando gotta come down. Only one spot, though. Magearna is as good as ever really -- and honestly more versatile than Lando-T these days with the invention of sets like phys def, weird cm pain split sets, or you can just stick to tried and true AV / 3 atk SG. This is more of a note for Magearna as I read it back... but as Lando-T's biggest fan I've got to comment here.)
Toxapex
S- Rank
Gliscor (Gliscor has usurped Landorus-T in usage for a reason. I don't rate it higher still due to the fact that it is less versatile and can be abused a bit harder in the builder naturally. That being said... it's just so much value in a mon. SD is great and pairs amazingly with trappers. U-Turn hazard control is great and pairs amazingly well with breakers that are hard to switch-in in such an offensive meta like Mawile.)
Kartana
A RANK
A+ rank
Tornadus-Therian
Greninja +
Ash Greninja
Ferrothorn
Clefable
Heatran (Heatran has lot an immense amount of steam with the prevalence of Gliscor and the shift to psychic types all being able to dumpster Heatran. At the end of the day though... it's Heatran. Spdef will always get at least some value and you can always use it as a one-for-one Z-nuke to bait Gliscors, Latis, Tyranitars and such.
Mega Latias (In my mind, the battle for best mega is between Latias and Tyranitar. I think Weavile's rise in prominence is almost enough for me to eke out Tyranitar, but Latias is always just so effective in any game SOMEHOW. While I don't personally enjoy playing the "french team" (Lati Tran Ferro Clef Glis Rotom), Latias is a huge cornerstone on one of the most consistent teams in SM history.)
Mega Tyranitar
Tapu Lele (All of my teams end up with a specs lele and I cannot stop myself. No mon makes progress faster. It is somehow the best lead in the tier because it's the safest way to get it in.)
Mega Alakazam
A rank
Rotom-Wash (It amuses me how some teams can just autolose to this mon. It lowkey invalidates all of those Aggron+Wish, Venu+Chans, etc type archetypes while also always holding value versus the best archetype of the tier. Next SPL I probably rank rotom mid A+.)
Mega Latios
Mega Mawile
Tapu Koko (I was super optimistic that Koko was good in this minimal-grass meta SM... it's just a bit underwhelming still. Not too many archetypes I like it on either. Screens is ok, and other than that I only really appreciate it on teams with abusers that can get in on uturns versus grounds and the rare grass types [Medicham]).
Tapu Fini
Serperior
Mega Lopunny
Garchomp (Long gone are the days of Garchomp being a top 3 Z-move holder in the tier. It still functions fine as a breaker for vanilla/bulky offenses -- the spl usage stats just indicate where teams value their ground type and z-user to be though.)
Mega Medicham (Not the mon for me, just a bit too mind-game and RNG reliant to win games. Empo using Medicham is an S- though.)
Magnezone
Excadrill
Chansey (OP. It's ironic how Chansey is one of the most pressuring mons versus offensive teams just through status, and being able to get free switches in to the mons that disregard status such as Toxapex.)
A- rank
Kommo-o (Kommo-o is just always so versatile on HO and vanilla offense(see: teams like Psy spam) as both a sweeper and a role compressor. It will never do anything truly amazingly though, as no sweeper set is perfect and the bulky sets can invite in a lot of unwanted threats that make headway, like Lele/Fini/Toxapex.)
Mega Scizor
Ditto
Tapu Bulu
Victini
Kyurem-Black
Weavile (I just really love the constraint Weavile can put on people in the builder AND in-game. It really is one of the most accurate mons in terms of "in practice" matches "on paper". Great on those Gliscor, Lati esque balances.
Celesteela
Volcarona
B RANK
B+ rank
Manaphy
Pelipper /
Mega Swampert (These two should never separate and thus rank together. Manaphy gets to be one spot higher because I think there is room to use it on webs/screens/some underexplored archetype -- generally will also just be another rain guarantee though.)
Mega Diancie
Gastrodon
Jirachi
Tyranitar
Slowbro
Hawlucha
Tangrowth
Mega Charizard X (Excited to see more Zard-X HOs pop up in the future. I personally saw an increase in value with scarf Lando-t being much more rare, and Gliscor being setup fodder to an extent.)
B rank
(Fulfills a pretty creative role on HO builds not looking to suicide with Lando-t. Doesn't have much defensive utility with the wisp/taunt/etc sets these days though, and is kind of asking to get outplayed.)
Kyurem
Zapdos
Hippowdon
Reuniclus (Usage probably doesn't reflect a B mon, but Reuniclus is a bit of a resource suck to ultimately be a mostly MU fish and "don't crit or freeze me XD" slot.)
Skarmory
Mega Sableye (Good players will beat Sableye teams. You have very little room to make your own headway and Sableye only fits on specific archetype.)
Hoopa-Unbound
B- rank
Amoonguss
Mamoswine
Mega Aggron
Mega Charizard Y
Mega Heracross
Bisharp
Alomomola
Gyarados-Mega
Hydreigon (Ngl this is the only mon here I'd use. It's just not that good though.)
Thundurus-Therian