Once players have finished the main story in a Pokemon title, chances are that they are either going to attempt to complete the Pokedex or jump into online battling, or perhaps even a combination of the two. Despite the fact that there are over 900 Pokemon in the series right now, not accounting for those missing from Sword & Shield, only a small number of those Pokemon are used online.
This largely comes down to the stats and abilities of certain Pokemon making them significantly better for competitive battling than others. Though, it also comes down to players becoming overly comfortable with a certain playstyle and with their favorite creatures, subsequently being unwilling or unable to try something new for a change and making some Pokemon become overused.
10 Politoed
For the longest time, weather teams, specifically Rain and Sun teams, have been great options for anyone to pick up and play no matter their experience in competitive battles due to their relatively easy playstyle. When it comes to Rain teams, the most common Rain setter is Politoed, which was also the first non-legendary Pokemon to have the Drizzle ability. As good as Politoed is, a new Pokemon gained access to Drizzle in Sun & Moon, Pelipper. Despite the fact that it is an arguably better Rain setter, as it not only benefits from the increase in Water-type attack damage but also the accuracy boost of Hurricane, as well as having great speed control options in Tailwind, Politoed is still the more common Rain setter and is somewhat overrated for what it can do.
9 Amoongus
The dreaded CHALK team that began life in X & Y may not have started Amoongus' life of stardom, but it definitely cemented it. An acronym for Cresseli, Heatran, Amoongus, Landorus-T, and Mega Kangaskhan, this team "core" was incredibly good, but also incredibly overused as a result. While CHALK isn't as viable in the current state of the game, largely due to the loss of Mega Evolution and the inclusion of several new Pokemon, Amoongus still sees a huge amount of play alongside fellow CHALK member Landorus-T. The Poison/Grass-typing is one of the most common in the series, but Amoongus seems to be the one that every player gravitates towards over other, similar Pokemon like Vileplume or Venusaur.
8 Dracovish
For a short moment in the history of Sword & Shield's online scene, it seemed like Dracovish was taking over and becoming the most used sweeper in the game, as its signature move, Fishious Rend, was a force to be reckoned with and was capable of tearing through even the toughest walls in a matter of seconds. That quickly fell apart, however, when players realized that it was a one-trick pony.
Despite this realization, however, Dracovish still sees a huge amount of play, even after the re-introduction of Kingdra into the game, one of the few other Pokemon with the same type combination, as well as the Crown Tundra DLC finally giving the fossil Pokemon access to their hidden abilities that make the other three more viable.
7 Togekiss
Togekiss has always been a great Pokemon, either in its Para-Flinch days before it gained the Fairy-type or in more recent years as a pain in the butt Dynamax Pokemon, but its overuse means that it overshadows many other great Fairy-types. While it does have some things that many other Fairy-types don't, such as access to Flying-type attacks or a more balanced spread of its defensive stats, its status as the go-to Fairy-type means that many others that are just as viable are left behind as a result.
6 Glastrier
Trick Room sweepers have been around for a long time, with many of them taking advantage of their amazing offense and defense that is normally overshadowed by their sub-par speed. Largely due to it being a brand new Pokemon, Glastrier has taken the place of other Trick Room sweepers like Snorlax and Rhyperior, who are just as good, just because it is new. Glastrier also seems to be the main Trick Room sweeper of choice despite the fact that it is an Ice-type, something that often turns players away from picking a Pokemon purely due to its type alone.
5 Porygon2
Porygon2 is probably one of the best users of the Eviolite held item in the series, but because it has been the go-to Pokemon for this role for so long, it tends to be overused in this regard. Galarian Corsola is just as good, though has less use offensively, but has largely fallen out of favor since Porygon returned to the game. Even Dusclops, which was the gold standard for bulky Trick Room setters in Sword & Shield, was dropped as soon as Porygon returned despite how well it was doing when it wasn't being passed over in favor of the duck from cyberspace.
4 Rillaboom
In all honesty, Rillaboom is probably the best new starter Pokemon in Galar, but it overshadows all others simply because of one attack: Grassy Glide. Libero Cinderace has just as much potential and Inteleon hits incredibly hard thanks to Sniper and Snipe Shot, but neither of them sees anywhere near as much use as Rillaboom.
This could come down to the fact that it functions as something of a mix between Incineroar and Tapu Bulu, one of the most used Pokemon in the series' history and a legendary Pokemon that so many players wanted to actually make work for the longest time, but its status means that it is taking the spotlight away from its fellow starter Pokemon.
3 Grimmsnarl
Prankster Pokemon have been go-to support Pokemon for a long time, as their ability to inflict status effects or using buffing moves like Tailwind, Reflect, or Light Screen before the opponent can act are something that all teams should be taking advantage of if they can. In Sword & Shield, Grimmsnarl was introduced as the Prankster Pokemon of generation eight, but while there are many other Prankster Pokemon that are just as good who returned to Galar, Grimmsnarl gets all the love for simply being newer.
2 Incineroar
Like with most starter Pokemon, Incineroar was fairly underwhelming when it debuted, but that all changed once its hidden ability, Intimidate, became available to players of Sun & Moon. It is easily one of the most overused Pokemon in the history of the series, though not without reason as it is a very good Pokemon, but its use, like Rillaboom, overshadows its fellow starters. There are also many other good Intimidate users in the game, but when a player is looking to add one to their team, their second stop seems to be Incineroar, right after the most used Pokemon in the entire series.
1 Landorus-T
Whenever Landorus-T is available in a Pokemon game, chances are that it will be on just about every online team in ranked battles. Landorus-T is a fantastic Pokemon with great stats, movepool, and ability in Intimidate, but its overuse often reaches the point of frustration for those who have to battle it over and over again. Like many other legendary trios, Landorus is far and away the most used of its group, with Thundurus and Tornadus seeing barely any play despite both of them having benefits that actually give it an edge over Landorus.