OP largely stolen from Hilos old CC
Welcome to the newly revamped UU Checks Compendium! Here, we'll look at each individual UU threat residing anywhere between the S and B- ranks of the current UU Viability Ranking Thread. The checks are ordered alphabetically, with the more niche Pokemon (C+ and below on the VRs) being placed at the end, after a |. The goal of this project is to determine which Pokemon that are viable in the UU tier can check each of these threats. Each Pokemon capable of checking a listed UU threat will be put into one of three categories depending on how well they can check the aforementioned threat. The three different categories are as follows:
Soft Check - a soft check is often a Pokemon capable of revenge killing the Pokemon it's aiming to check, meaning that it is often more of an offensive check and should almost never directly switch in. A soft check can also be defined by its ability to switch into some of an opposing Pokemon's commonly run moves, but not all, meaning that they only win if given a free opportunity to switch in. A good example of a soft check is Scizor to Terrakion, as it often can't comfortably switch into Terrakion's attacks without risking a KO or considerable damage, though with STAB priority Bullet Punch, it can handily revenge kill Terrakion once in play. In essence, a soft check isn't often capable of consistently switching into and walling an opposing threat, and is instead capable of beating it through revenge killing it, taking on specific sets, or winning a 1v1 situation only if given a free opportunity to come into play.
Hard Check - a hard check is a Pokemon that is often capable of switching into a beating an opposing threat, though there are still situations in which it can be beaten (i.e. if the Pokemon it aimed to check were to run an often unconventional coverage option to lure it in). Hard checks can also occasionally be defined as Pokemon capable of switching into and beating an opposing threat, but due to specific circumstances, only being capable of doing this once or twice in a match as opposed to consistently. Hydreigon is a good example of a hard check to Celebi, as it can often switch in on Celebi's offensive and defensive sets and win, but should fear coverage options such as Dazzling Gleam or Thunder Wave. Mega Aerodactyl is a good example of a hard check to Chandelure, as when healthy it can switch into and win, though it can only switch in about once throughout a match.
Counter - a counter is a Pokemon capable of consistently switching into an opposing threat throughout an entire match and winning against it in a 1v1 situation unless it is powered through by a rare Critical Hit or is at low health. Nihilego is a good example of a counter to Togekiss, as it can always freely switch in on Togekiss, and unless at fairly low health, will have an extremely high chance of beating it one on one. It's important to remember the distinction between a counter and a hard check, which is that a counter can consistently switch into any move from a Pokemon it aims to check and multiple times, whereas hard checks must look out for various coverage options and, in some cases, cannot switch into a threat multiple times.
Each of these sections will also be divided into two other subcategories of:
Offensive - Each of the three main sections will have a category determining which of the listed Pokemon are offensive checks to the said threat. An offensive check or counter serves the same process as the examples provided above but looks to overwhelm the threat via offensive pressure. An example being Mega Altaria is a hard offensive check to Krookodile as it looks to check it via pressuring it with STAB Fairy coverage than switching into it consistently.
Defensive - Same as the offensive checks, but this will inform you of the defensive answers instead. A defensive check or counter is a good switch into the threat and looks to beat it down through sheer bulk or slowly wearing it down with status. An example being Empoleon which is a defensive check to Latias who looks to wear it down with the combination of Toxic + Protect.
Some Pokemon are known to have different sets, so this will be used to refer to a defensive or offensive variant of it. For example, Primarina is known for its defensive set, so this will signify that set while under the offensive category it will be referring to its offensive Choice Specs set.
There might be exceptions to the above rules, so these will be handled on a case-by-case basis. Hopefully, this thread will be useful to help you to determine what offensive UU threats you need to consider when team building. The community is encouraged to get involved by making suggestions to this project if they think something else should be considered as a check or counter instead. If you do wish to inform me of potential improvements for this thread, please refer to the Viability Rankings to make sure it is viable in the UU metagame first. I will look to keep this updated often to be as accurate as possible!
Welcome to the newly revamped UU Checks Compendium! Here, we'll look at each individual UU threat residing anywhere between the S and B- ranks of the current UU Viability Ranking Thread. The checks are ordered alphabetically, with the more niche Pokemon (C+ and below on the VRs) being placed at the end, after a |. The goal of this project is to determine which Pokemon that are viable in the UU tier can check each of these threats. Each Pokemon capable of checking a listed UU threat will be put into one of three categories depending on how well they can check the aforementioned threat. The three different categories are as follows:
Soft Check - a soft check is often a Pokemon capable of revenge killing the Pokemon it's aiming to check, meaning that it is often more of an offensive check and should almost never directly switch in. A soft check can also be defined by its ability to switch into some of an opposing Pokemon's commonly run moves, but not all, meaning that they only win if given a free opportunity to switch in. A good example of a soft check is Scizor to Terrakion, as it often can't comfortably switch into Terrakion's attacks without risking a KO or considerable damage, though with STAB priority Bullet Punch, it can handily revenge kill Terrakion once in play. In essence, a soft check isn't often capable of consistently switching into and walling an opposing threat, and is instead capable of beating it through revenge killing it, taking on specific sets, or winning a 1v1 situation only if given a free opportunity to come into play.
Hard Check - a hard check is a Pokemon that is often capable of switching into a beating an opposing threat, though there are still situations in which it can be beaten (i.e. if the Pokemon it aimed to check were to run an often unconventional coverage option to lure it in). Hard checks can also occasionally be defined as Pokemon capable of switching into and beating an opposing threat, but due to specific circumstances, only being capable of doing this once or twice in a match as opposed to consistently. Hydreigon is a good example of a hard check to Celebi, as it can often switch in on Celebi's offensive and defensive sets and win, but should fear coverage options such as Dazzling Gleam or Thunder Wave. Mega Aerodactyl is a good example of a hard check to Chandelure, as when healthy it can switch into and win, though it can only switch in about once throughout a match.
Counter - a counter is a Pokemon capable of consistently switching into an opposing threat throughout an entire match and winning against it in a 1v1 situation unless it is powered through by a rare Critical Hit or is at low health. Nihilego is a good example of a counter to Togekiss, as it can always freely switch in on Togekiss, and unless at fairly low health, will have an extremely high chance of beating it one on one. It's important to remember the distinction between a counter and a hard check, which is that a counter can consistently switch into any move from a Pokemon it aims to check and multiple times, whereas hard checks must look out for various coverage options and, in some cases, cannot switch into a threat multiple times.
Each of these sections will also be divided into two other subcategories of:
Offensive - Each of the three main sections will have a category determining which of the listed Pokemon are offensive checks to the said threat. An offensive check or counter serves the same process as the examples provided above but looks to overwhelm the threat via offensive pressure. An example being Mega Altaria is a hard offensive check to Krookodile as it looks to check it via pressuring it with STAB Fairy coverage than switching into it consistently.
Defensive - Same as the offensive checks, but this will inform you of the defensive answers instead. A defensive check or counter is a good switch into the threat and looks to beat it down through sheer bulk or slowly wearing it down with status. An example being Empoleon which is a defensive check to Latias who looks to wear it down with the combination of Toxic + Protect.
Some Pokemon are known to have different sets, so this will be used to refer to a defensive or offensive variant of it. For example, Primarina is known for its defensive set, so this will signify that set while under the offensive category it will be referring to its offensive Choice Specs set.
There might be exceptions to the above rules, so these will be handled on a case-by-case basis. Hopefully, this thread will be useful to help you to determine what offensive UU threats you need to consider when team building. The community is encouraged to get involved by making suggestions to this project if they think something else should be considered as a check or counter instead. If you do wish to inform me of potential improvements for this thread, please refer to the Viability Rankings to make sure it is viable in the UU metagame first. I will look to keep this updated often to be as accurate as possible!
Tsareena
Aerodactyl-Mega, 
Beedrill-Mega, 
Bewear, 
Cobalion, 
Chandelure, 
Doublade, 
Gengar, 
Heracross, 
Houndoom-Mega, 
Hydreigon, 
Infernape, 
Kyurem, 
Latias, 
Lucario, 
Manectric-Mega, 
Muk-Alola, 
Nidoking, 
Nidoqueen, 
Nihilego, 
Roserade, 
Scizor, 
Terrakion, 
Volcanion | 
Metagross, 
Pidgeot-Mega, 
Salazzle, 
Tornadus, 
Toxicroak, 
Venomoth
Rotom-H | 
Articuno, 
Silvally-Steel
Moltres | 
Crobat, 
Talonflame
Aggron-Mega, 
Altaria-Mega, 
Togekiss
Amoonguss | 
Chesnaught