Tier 1
Sand Rush Excadrill, under Sand (and Tailwind [for extra speed control]), can outspeed most mons with unboosted speed, allowing it to potentially OHKO its target(s) before they can OHKO it. It can use several items, such as: Life Orb, Focus Sash, Choice Band, and etc. It also has the 2nd highest ATK out of all the Ground-type mons that cab be used. Overall, it's a very strong and fast threat that should be ranked higher, imo.
(252+ Atk Choice Band Excadrill High Horsepower vs. 252 HP / 0 Def Melmetal: 386-456 (81.4 - 96.2%) -- guaranteed 2HKO)
(252+ Atk Choice Band Excadrill High Horsepower vs. 0 HP / 0 Def Venusaur: 306-361 (101.6 - 119.9%) -- guaranteed OHKO)
(252+ Atk Choice Band Excadrill High Horsepower vs. 0 HP / 0 Def Zeraora: 666-786 (210 - 247.9%) -- guaranteed OHKO)
Milotic is the Incineroar counter, plus more. Right now, Sleep is extremely prevalent, and one of the mons putting other mons to sleep happens to be Milotic. Use Coil twice, and your Hypnosis has 100% accuracy. It's bulky, so it's hard to OHKO (even for mons like Zeraora and Venusaur). If you want a powerhouse, you can use offensive moves like Hydro Pump and Blizzard without the drawbacks. Recover allows it to stay on the field. Tbh, this mon is extremely hard to break.
(252+ SpA Life Orb Venusaur Giga Drain vs. 252 HP / 0 SpD Milotic: 242-289 (61.4 - 73.3%) -- guaranteed 2HKO)
(252 Atk Life Orb Zeraora Plasma Fists vs. 252 HP / 252+ Def Milotic: 322-382 (81.7 - 96.9%) -- guaranteed 2HKO)
Venusaur is the second premier mon using Sleep rn. Chlorophyll (under Sun [and/or Tailwind], like Sand Rush, Venusaur can outspeed most mons with unboosted speed. While the Sleep set is more common, Venusaur works well with Growth (and gets double the boost) and it can even use Sunny Day to set its own Sun (making a Sun setter unnecessary). Outside of Sun, Venusaur can easily take getting hit by Fire-type moves, and potentially strike back with Earth Power (if the mon is Fire-type).
(252 SpA Life Orb Charizard Heat Wave vs. 252 HP / 4 SpD Venusaur: 265-315 (72.8 - 86.5%) -- guaranteed 2HKO)
(0 Atk Incineroar Flare Blitz vs. 252 HP / 252 Def Venusaur: 260-308 (71.4 - 84.6%) -- guaranteed 2HKO)
Zeraora is extremely fast, has speed control options, can use Fake Out, and has a diverse range of moves it can choose from to counter its checks (like Excadrill). Volt Absorb allows it to eat moves from another Zeraora and heal. Zeraora is the main check to Milotic and Blastoise, and both are very bulky mons. It even has a decent bulk. You can't go wrong using this mon.
(252 Atk Choice Band Zeraora Plasma Fists vs. 252 HP / 180 Def Blastoise: 372-438 (102.7 - 120.9%) -- guaranteed OHKO)
(252 Atk Choice Band Zeraora Plasma Fists vs. 252 HP / 0 Def Milotic: 536-632 (136 - 160.4%) -- guaranteed OHKO)
Tier 2
Dragapult really isn't that good. It has nice abilities, moves, and speed, but it just doesn't do as much as it needs to. Mons like Incineroar and Zeraora carry Knock Off, so it can get really hectic for Dragapult. Also, Dragapult is kinda frail, so it'll be lucky to get an OHKO before being dropped (and the only thing saving it from that is its speed).
Very good mon. As many have said before, Fake Out + Follow Me is nasty. I'm really nominating Blastoise for 2 because of its Shell Smash set. All you have to do is use Shell Smash with White Herb, and then you end up with a Blastoise that's fast, strong, and can tank hits. Water Spout legit puts everything to sleep (permanently) by being so strong, and, under Rain, Rain Dish allows it to gain help (which, with Leftovers and Aqua Ring, can be used to keep Water Spout at maximum damage.
Keldeo, with Choice Scarf and some speed investment, can outspeed and OHKO Excadrill. I was looking for a fast Excadrill counter, and Keldeo happened to be one of them. Keldeo is fairly strong, and also works well with Assault Vest. It's diverse movepool allows it to check its checks (like Venusaur), plus more. Keldeo should be in the same bracket as Milotic and Blastoise.
I've been using Necrozma as a TR counter, and it never fails. It's too bulky to be killed by the TR setter, bulky enough to activate Weakness Policy, and has plenty of moves to check a variety of stuff (like Heat Wave for Melmetal or Earth Power for Incineroar). Necrozma is, imo, the best Psychic mon rn. Gothitelle is too weak, and Mew doesn't even use Psychic like that.
Not sure how this went from 1 to 3, but please increase this. Have a problem with status moves? Np. Just use Whimsicott with Safeguard. Focus Sash is gonna allow Whimsicott to set up at least one thing before it dies, and its Tailwind is the fastest. I've won so many matches because this thing allows my mons to go first. It can even do some damage to Milotic and Blastoise (among others).
Tier 3
Clang clang. But no, for real. Kommo-o is pretty decent rn. Not many Psychic, Dragon, or Fairy mons to check it, it has access to Clangorous Soul (which can make it a monster), it has Soundproof for mons like Sylveon or Overcoat for mons like Venusaur. Did I mention it learns Flash Cannon and Flamethrower, so it can also check the aforementioned mons. Use it with a mon with Heal Pulse (like Gothitelle), and your opponent has a problem. Once this mon gets set up, it's hard to kill unless you have a super effective move.
Fake Out is the main reason for me wanting to see this thing increased. Incineroar, Zeraora, Mew, Blastoise, and etc all use Fake Out, and many people use more than one of those mons on their team. That being said, Tsareena just stops all the madness. If she's not needed, she can use U-Turn to pivot out. Trop can allows for ATK control and Low Sweep allows for SPE control. She even has access to Rapid Spin, to remove hazards like Toxic Spikes, and Knock Off, to remove its victim's item. I use this mon with Choice Band, and I haven't looked back. Milotic and Blastoise can't take getting hit by this mon too much before dying.
Tier 4
Ninetales is a better Sun setter than Torkoal rn because TR is disappearing. It can power up its own Fire-type moves, can check Water-type mons with Solar Beam, and Hypnosis is pretty good rn. Ninetales speed is decent, as well, allowing it to outspeed a lot of mons.
Tier 5
Obstagoon is my favorite Gen 8 mon. Defiant is amazing rn, and this thing can OHKO Incineroar (with Choice Band) after being boosted once. Its speed is nice, and its bulk is okay. It can also use Fake Out to slow the opponent down and Parting Shot to control its threat's offensives, as well as Obstruct to decrease the DEF of those who attack into it. Did I mention you CAN'T use Parting Shot on this mon, and that it also drops Tyranitar and Excadrill with ease (using Close Combat)? It doesn't have many threats rn, so I'm definitely taking advantage of this mon.
Idk why this thing isn't ranked. Barraskewda is the best Swift Swim user we have when it comes to Rain. It can outspeed and OHKO shit like Zeraora (with Drill Run) and Excadrill (with Waterfall). Even at -1, if given a Choice Band, Barraskewda can still drop Incineroar. It can use Psychic Fangs to counter Venusaur. Tbh, the attack, speed, and movepool of this mon makes it great rn, and could put Rain teams in a very good position. Barraskewda works perfectly for a late game sweep.
With Unburden and some speed investment, this things can outspeed Sand Rush Excadrill, and OHKO it with a Fighting-type move. I'm using White Herb on it to counter Incineroar (since it resets lowered stats to 0 and Hawlucha is a Fighting-type mon). Since Hawlucha is also a Flying-type mon, it has the potential to OHKO Venusaur.
Malamar, with Scope Lens, Night Slash, and Psycho Cut, is able to check mons like Mew and Venusaur, while also potentially hitting them with a critical hit. Contrary and Superpower makes it a good Incineroar counter, and then you can use Baton Pass to give all of Malamar's stats to another mon without any drawbacks. It doesn't have any resists, but not many Bug-type and Fairy-type mons exist to counter it.
I've been seeing this thing a lot since Toxic Spikes rose in popularity. Not only can it set Toxic Spikes, but with Merciless, it can drop mons that are poisoned with ease (especially if it has Life Orb [which it can mitigate the recoil by recovering). Baneful Bunker is good, and Venom Drench is even better. Depending on how you use your Toxapex, it can counter Fire-type and/or Fairy-type mons (like Incineroar and Togekiss). Toxapex's bulk is gonna guarantee it gets to set up, and it can cause a lot of problems if it starts boosting its stats with moves like Stockpile.