S:
Tyranitar-
Obvious. Sand makes stall beatable. Playing adv without sand is boring and uncool. Also: ddtar is incredible right now, and if you set it up right it cleaves through all those spinner teams that rely on suicune or milotic to beat it. Suittar is great, although it's not actually that good at removing gengar in my experience. Very good to deal that one hit to something like aero or cbmence that puts it in range of your other attackers though.
A+:
Zapdos-
Spdefzap is amazing, and offensive is of course quite good too. Zapdug seems kinda flavor-of-the-month to me, but I don't think it'll drop off the face of the earth either. Spdefzap is just annoying as shit when you don't need it to wall special attackers. It's not incredible at doing that, but it will duel mixmence and come out on top reliably, and as long as you're not asleep it's fine against Moltres.
Swampert-
Nothing else in the tier can reliably beat dd tyranitar and aerodactyl in the same battle. Even pert struggles to do that sometimes, but it definitely puts up more of a fight than something like Milo or Suicune. Also, nobody runs offpert anymore, so offpert is good. Be sure to run some HP or defense investment though, max HP max Spatk won't fly with how good aero is at the moment. Still never gonna live 2 banded edges, but pert can be a really serious offensive threat vs some teams. Monopert is also quite good vs spdefzap that isn't running hpgrass, as forcing it to rest wins half of the battle. Endpert still good. I keep thinking I'm done writing this one but still coming back to it, I just also want to say that on a lot of teams it's nice to run speed creep on defensive pert to outspeed Blissey.
Celebi-
Maybe a bit of a hot take, but Celebi is my go-to special tank right now. Blissey and Jirachi have their own advantages (wish passing, better status spreading, although I have seen toxic Celebi I don't think you're normally going to have the move slots for it. Superbi is great as always, especially if you can remove dug or punish it for trapping you. Not too much more to say here: Celebi is just extremely good right now.
Skarmory-
Spikes are the best and skarm's the best spiker. Having the ability to stay in on something when you don't know whether or not it's carrying a fire move is quite good. Taunt is a really great tool for preventing other spikes, preventing refresh, preventing blissey's recovery, etc. Also, most of skarm's switchins don't really like taking a toxic. Magneton usage is low, and even when it's high skarm is great.
Metagross-
Second-best rock resist in the tier. Mixed (both psychic/fire and electric/grass are great) is really good, physdef is great, agiligross is underwhelming imo but has some great matchups, CB is great with a little bit of support, and you really can't go wrong with lefties 4atks. Busting out the protect when your opponent doesn't know you have it can be devastating as well.
A:
Blissey-
Zapdug doesn't mean Blissey's not still broken. If you don't let this thing get trapped in the first 10 turns of the battle, you can expect it to put in quite a bit of work. Toxic wreaks havoc vs opposing balance (especially with snatch! fuck milo), twave is very very good vs offense, wish helps out certain teams immensely and will consistently put in good work regardless of the matchup. People on ladder never scout fire blast with their forre either, although that's not exactly something you can count on in tours when people play more intelligently.
Jirachi-
I guess #8 Jirachi is a bit of a hot take right now, but it seems at home to me right between Blissey and Gengar. Astarachi is really good of course - essentially combining two Blissey sets into one mon (being able to pass wish and paralyze, with the bonus of some chance to burn) with the addition of a toxic immunity and rock resistance. What's not to like? Well, as is the nature of role compression, rachi doesn't particularly stand out at performing any of those roles. As a sole rock resist it's almost laughable, and it has a crippling weakness to being burnt, getting fully paralyzed at an inopportune moment, or just plain getting set up on by something behind a substitute. Not to say that those are unjustifiable flaws, but they're there to be considered. Superachi has its home as a cmspam staple, and cm wish thunder ice punch (or psychic fire- being blanked by ttar is a liability though) is one of the most incredible punishers of fat do-nothing teams in the tier.
Gengar-
A must-have for offensive spikes teams (and if you're looking at these vrs and can't tell what my preferred style of team is, then I don't know how much I can help you). Gengar is a massive jack-of-all-trades: just randomly generate four of its legal moves and you've probably got the next big gengar set. 3atks boom, 3atks wisp, taunt wisp boltbeam, even fucking hypnopunch if you really don't care what other people think about you. Spdef giga is great to fuck with all these forre suittar teams running around.
Salamence-
Mixmence is so, so, so good right now. It keeps fighters at bay for long enough that you should have time to pick apart the rest of your opponent's team, and it provides great offensive presence on its own as well. Particularly with spikes, as this thing forces switches like nothing else. Well, except Gengar. And Aero too I guess. Point is, if I've got a team of five that I think looks fine so far, it's probably pretty fighter-weak and I'll probably end up putting mence in that last slot.
Suicune-
I've been pretty low on Suicune for a while now, as it always seems like a bit too much of a matchup pick to me. It's just a mon that has a really hard time keeping pace against offense, and you're not nearly sturdy enough (or powerful enough) against ddtar and aero for my liking. You really have to keep this thing essentially at full and awake if you want it to check those guys long-term. I know cune out of the top 10 probably looks like heresy to a lot of people, but this ranking is still pretty high if you think about it. That's mostly due to a moderately fast set with a bold nature, roar, and something else on the team to act as the primary rock-resist. It's an excellent stopgap to opposing suicune, and it's a monster in the endgame. Sleep talk isn't really missed because you pull rest 80% of the time anyway.
A-:
Aerodactyl-
Crazy good mon. Use with spikes and something to damage pert and/or meta and this thing is insane. 99% of the time you're not going to be left thinking 'gee, wish I didn't have aero last at this moment'. Plus, nothing's able to BS out a win like last-mon aero.
Dugtrio-
Trapping is broken. Zap-partnered beat up shenanigans improve dug a little bit, but all it really does is make the trap on Blissey that much more reliable. Luckily for ADV dug, it's pretty abusable and doesn't allow you to build and play like a (complete) moron and get away with it, so it's almost certainly here to stay. Everyone knows what dug does and why it's here.
Forretress-
I'm torn between the top of this tier and the bottom of the next one. Forre is certainly good, and can put in a lot of work with the proper support. The problem with forre is it's just not as good at getting its spikes down as skarm is. Great matchup vs skarm teams that aren't prepared for it, but it's not exactly the hardest mon in the world to prepare for. Forre + dol is really good, as we all can probably tell by watching this SPL. Otherwise, you're going to need reliable pursuit. HP ghost or zap cannon on forre is pretty cool to dissuade gengar from coming in on it, but you're already quite starved for move slots (spikes, spin, eq, bug, boom, and you're trying to fit one that's not those?). Counter, toxic, and protect aren't too bad either, but you probably just aren't going to have the moveslots realistically.
B+:
Starmie-
Defensive has fallen off a little bit due to astronomical suittar usage, but it's still pretty dang good. It feels sketchy to rely on this to actually beat physical threats sometimes, and other times it feels unbreakable. Also, try out toxic on boldmie if your team can handle the sauce. Being able to force Blissey to switch is a pretty nice boon. Not too much to say about offensive starmie, it's an excellent cleaner for spikes, dugtrio, or cmpass special offenses.
Claydol-
A moderately better spinner than Starmie, but a poor offensive presence and an underwhelming defensive presence is going to land it lower. Boom is a great move. Don't fall into the trap of running this as your primary rock resist, though. Calcs against aero are a very sad sight indeed, and this thing is far from a good counter to ddtar.
Moltres-
Try switching into modest flamethrower with spikes up. It's not happening if you're not blissey. A fun interaction is milotic trying to come in on you with two spikes up, taking the wisp, and you can just click hp grass there because it's not like surf is an ohko and milo can't safely click anything other than recover. You'll crit it eventually, or they have to switch out and they're still burned (or at pathetically low health). I don't think this thing really needs roar, so feel free to drop that for sub, overheat, toxic, sunny day, whatever you want.
Heracross-
If megahorn had 100% accuracy then this thing jumps up a whole tier. This thing is just so ridiculously strong, and theoretically is the best cleaner in the tier. Unfortunately, pokemon is a messy game and you likely won't hit all your horns. CB is good but prediction-reliant, subsalac is great (try with focus punch), sub 3atks/sd 3atks/4atks are all fine, just held back by accuracy and not really doing anything if you click the wrong move.
B:
Snorlax-
I think utility lax is not good. You come in on spikes and just get 3hko'd by zapdos. You're outsped by everything and just do next to nothing if you guess the switchin wrong. Can put in work if you guess right, but what can't? Curse sets actually aren't bad (curse boom is fine and curse rest is acceptable if you have a ton of support)
Milotic-
One-trick pony, crumbles to especially strong hits. Kills mixed offenses like nothing else though, and can reliably 1v1 any bliss set (except snatch! fuck milo). Good molt switchin if you can keep spikes off, and better than suicune on stalls because you won't be asleep.
Porygon2-
I think more highly of p2 than most people, probably. It's definitely usable. Reverse trapping duggy sets up some stuff to be abused, and it's a good all-around check to most salamence sets. My preferred set is toxic boltbeam, but I think twave hpfire is also good as a mence lure, and hp fighting is good as well although harder to ev and fill out a moveset.
Breloom-
Sleep is broken as fuck but this thing is too flawed in my opinion to really abuse it. If someone makes hp bug loom work then maybe it moves up but I just have a hard time seeing it. You really need all 3 of those fucking fighting moves. I think stun spore is good over focus punch, or you can run sd there too if you want.
Vaporeon-
If there's anything I'm willing to call the 'next big thing' it's this. Vap is an amazing offense enabler. Staring this thing down with your ttar turn one, knowing if you go to Blissey you're going to let them pass a 101 hp sub to some fighting type but also knowing that pump massacres ttar and 2hko's the rest of your mons is terrifying. Great case for why roar cune is good - except, also, it completely walls suicune, so that's not that great either. Vaporeon hits way harder than you think it does, I promise. Don't use defensive though, that's garbage.
Magneton-
Mag offense is bad and magdol is worse. Skarmmag is pretty good (spikes + a slot for aero, how bad can that be), but it's not exactly on top of the meta right now. skarmmag probably beats forre stuff pretty easily I guess but I can tell you that I've never played that matchup and don't really intend to.