S
I think latios has established itself by now as the best mon in the tier, and the development of colbur sets have only made it better.
Tyranitar second as its defensive utility is unmatched. Best check to latios, checks magic guard users, sand suffocates rain.
Ferro rounds out S rank due to its ability to be used on both rain and sand to equal success. Spikes are massive in this meta, and ferro is arguably the best user. Also helps absorb a few attacks from latios.
A+
Gliscor to the top of the A tiers due to its great performance recently, with landorus falling from s to take the spot behind it. Every sand team basically needs one of the ground/flying mons, as keep physical threats such as excadrill in check. Gliscor is bulkier in practice due to poison heal and the tendency to invest fully in bulk. It can be a great sr setter with the ability to poison stall, pivot via uturn, or knock off items. This set can also offer some defense vs. volcorona, which is hard to come by on many sand spike teams. On the other hand, gliscor is a dangerous sweeper with swords dance. If a team doesn't have a check to sd gliscor, they lose on the spot, and teams with limited counterplay often lose long term. Great mon, borderline S.
Landorus on the other hand is still a great pokemon. It's still great, but competes directly with gliscor for a spot.
excadrill - every team needs hazard control, and excadrill is the best of the bunch. matches up decent vs spikers, beats every spinblocker. very threatening in its own right with sand force. arguably better than landorus, but the prevalence of psyspam as the most common sand variant holds it back from being a necessity.
zam - psyspikes are probably the best form of team in the meta with zam at the forefront.
thundurus-t - probably the biggest surprise here, ranked above politoed. thund-t is the primary reason to run rain, as well as simultaneously being one of the biggest threats to rain. The only thundurus checks in the tier that sees any use are latios, spdef celebi and possibly gastrodon. Latios can only switch in once, as hp ice 2hkos, and it also risks a thunder para. Gastro loses to grass knot and has a tough time with nasty plot. Celebi is probably the best of the bunch, but even that loses to np or a rare u-turn. The inability to be walled, decent speed tier, set up moves in agility / nasty plot / substitute all lead to a top pokemon.
keldeo - had this at top of a, but fits on both rain and sand. premier offensive threat
A
garchomp is reliable but doesn't excel at one thing imo. great sr setter, good scarfer, dangerous sd sets, mix set are all good, but none of them are outstanding individually.
loom has more upside than chomp, but more downside too. a good matchup with loom can have an opponent unable to touch it. doesnt do much in bad mus. must be respected in teambuilder
toed - low by some people's standards. rain doesn't hold up well in the sand vs rain mu. with proactive play and intent to keep hazards/weather up at all possible times, sand will be on the field more often than not, making it difficult for the rain player to do much of anything. by itself, one of the least useful mons in ou. team structures can have difficulty fitting checks for top ou threats ex thundurus, latios. despite all the hate, enables an entire relatively successful archtype.
reuniclus - similar to loom. necessitates answers in teambuilder, can have less than impactful games/win them outright. good answer to other psyspikes.
volcorona - due for a big raise. biggest hit/miss mon on the list, but more hits than misses and the matchups it gets are fantastic. Requires a certain build, but is the best sweeper in ou.
skarmory - great spiker on sand, due for a raise. better mu than ferrothorn vs excadrill. alakazam check, takes advantage of ground/flyings. good on ho w/ jellicent. weakness to rain (without jellicent) keeps it from going any higher
tentacruel - not big on tentacruel. its a spinner that is pretty easily spinblocked by the only viable ghost in the tier. Bad mu vs psychic heavy sands. however still a roadblock that needs to be overcome in order to beat rain, and has some nice qualities of its own.
A-
don't think rotom-w is a meta defining force anymore. too easily chipped with sand, sr, u-turns, burns, etc. still an effective pivot for a time, and can be hard to switch into. ok scarfer.
mamo - fantastic mon. stab combo hits a large portion of ou super effectively. ice shard is useful for faster threats, but its speed is still ultimately a let down.
dnite - 2nd best setup sweeper to volc. if volc has a poor mu, hyper offensive teams usually rely on dragonite to do some heavy lifting. different items can change the checks it has. great volcorona check of its own. sub dd is decent as well
bit of a power gap here from the first half of a-. terakion is very hard, if not impossible to wall, but relies on 80 acc stab far too much. it has great bulk, but is weak to spikes. its ultimate downfall for me is that it just doesnt seem to fit on very many teams. still a threat though
magnezone - bad mon, but it takes care of the spikers which can be huge in a tier so centered on hazards. facilitates its own team style (dragmag).
sometimes, teams don't mind trading skarm/ferro for a layer or two of hazards. can be dangerous when paired with sd gliscor. decent overall and can make a big impact.
heatran - on paper, heatran is awesome. However in practice, the hazards weakness and rain susceptibility are a big knock. Very hard to wall, but 78 speed isn't great. He also suffers the same issue as terakion in that its hard to find a spot for heatran on a team. In this case due to poor synergy with the tier's hazard control.
B+
jirachi - hard to fit. not very effective offensively, defensively hazard weak and faces competition w/ other steels. decent on rain.
celebi - nice water resist, good defensively. sr. nasty plot can make a decent sweeper
gastrodon - water immune that isn't pursuit weak. checks some physical threats decently, such as dragonite and landorus-t. spikes weak hold it back, but best of the "bulky waters" imo.
jellicent - only spinblocker. great on offensive spike teams. hard to fit defensively, weak to hazards and doesn't spinblock excadrill more than once. almost guarantees a win vs rain. outside of its beneficial mus, can have a tendency to be mediocre
tornadus - very strong, but hard to use. protect usage is high, ttar are always chople or scarf, spdef skarm relatively common. hard to fit on rain with prevalence of thundurus-t.
starmie - best offensive spinner. ultimately kind of bad, but fills its niche. specs on rain is fun
kyurem-b - strong, but easily played around in practice. still can guarantee a ko. mix sets are somewhat viable
scizor - decent sweeper that helps defensively on ho. fills holes on rain
B
seismitoad - good movepool, water immune, best niche bulky water imo. no recovery hurts sometimes, but should be played on teams that need it as a pivot
slowtwins - both viable, but compete with gastro/jelicent/seismitoad. slowbro helps secure physical mus for sand, while slowking can ward off keldeo. Offensive sets are a thing, too.
mew - I want to love mew but it is so hard to get to work. weak to strong water moves in rain, but sand mu can be good. relies on wisp. hard to fit on a team in the end.
gyarados - devastating sweeper in optimal conditions, which are ultimately pretty hard to set up. can easily win games, but will often do nothing.
salamence - decent scarfer. demands a quick response or can get out of hand. drag mag is pretty bad though.
milotic - generic bulky water. offers refresh and recover, but the other waters have a more well defined niche.
kyurem - another mon that is better on paper. too hazards weak, but ice stab is a valuable thing to have. B for its potential, B- for how it often performs.
B-
xatu - does a decent job of keeping hazards, especially spikes, off the field. can have trouble with more offensive sr setters, or pivoting until it can be picked off. pretty good but forces awkward team compositions imo.
latias - sub cm can devastate rain, but there really isnt much of a reason to run this. better options for wearing down tar/ferro exist than dual lati.
mienshao - underrated. play around obvious protect turns for a big advantage! prevalence of psychics hurts it though.
ninetales/cresselia/chansey - sun stall package, all mons go together. sun is very hard to get to work, but this would be the package to run if you try it. very hard to kill in sun, but not worth playing due to the difficulty in maintaining the weather.
hydreigon - eh. doesnt really do anything in practice. speed tier hurts it a lot vs latios. still potentially has its uses
cloyster - ice stab is very threatening vs traditional sand builds, and a mon that can get free setup vs gliscor/landorus always has some use, even after the kings rock ban.
hippowdon - decent wall, good sr setter. hard to fit on a team (dual sand?)
toxicroak - not very good but has the opportunity to make an impact, mons below it don't really do much