The Grass-type starter of Galar and, arguably, the best option competitively between the three Galarian starter Pokémon, Grookey, and its evolutions, are always a good pick for a standard Sword & Shield playthrough. Between its defensive bulk and offensive capabilities, particularly when it has Grassy Surge as an ability, it's will make quick work of any battle the player might face.
Grookey shouldn't be working alone, however, as the player is given six slots for team members for a reason and should aim to build a team around their starter for maximum efficiency and battle capabilities.
10 Catch: Sizzlipede
This Bug/Fire-type Pokémon can be quite hard to find, though it is well worth the hassle considering how early you can catch it. While it only has a 1% encounter rate in shaking grass on Route 3, it can be much more easily caught in the Motostoke Gym where catching Pokémon is part of the challenge. Bug and Fire-type attacks cover Grookey's Ice-type weakness as well as deal with the majority of Grass-types, though it also shares in its Flying-type weakness. With this in mind, other members of your team should include a Pokémon that can handle Flying-types, as the Corvinight line is used by quite a few trainers.
9 Avoid: Rolycoly
Found just after Sizzlipede, it can be tempting to catch this much easier to find Fire and Rock-type Pokémon rather than struggling to find the former. However, despite Rolycoly coving Grookey's Fire, Flying, Ice, and Bug-type weaknesses, it also falls short outside of a very specific doubles strategy involving Weakness Policy and Steam Engine. Nearly every other Fire-type in the game is a better choice in a standard playthrough, while several mid-game Rock-types, such as Larvitar and Roggenrola are much better Rock-type options than Rolycoly's line.
8 Catch: Wingull
Wingull and Pelipper spent a long time as mediocre Pokémon, as, aside from its decent type combination, it didn't have a lot going for it. Once it was able to have the Drizzle ability, which sets up the Rain weather condition when sent into battle, it not only get a great boost in power but also a usefulness on Rain centric teams.
Wingull and Pelipper not only deals with opposing Fire, Bug, and Grass-types but also sets up the Rain weather condition to make Fire-type moves weaker and gives the player access to powerful support moves like Icy Wind and Tailwind, the latter of which not being available on nearly as many Pokémon in generation eight and helps improve Grookey's otherwise slow speed.
7 Avoid: Tympole
Before the likes of Kingdra were introduced to the game, as well as the availability of Beartic's hidden ability, the Tympole line was considered the only decent Swift Swim user to be paired with Pelipper. While it not only has a useful Ground/Water-type combination that is only weak to Grass-type attacks, but also combos with Pelipper very well, it falls short in several key areas, mainly on its reliance on the Rain being active and its relatively low stats compared to most late-game Pokémon. On a dedicated Rain team, Tympole's line has its uses, but in a standard playthrough, it's better left alone.
6 Catch: Gastly
While you can also simply wait and catch the overworld Gengar spawn later in the game, catching a speedy Ghost/Poison-type that hits very hard is useful against just about every trainer in the game before this point. Aside from Piers, the Gastly line is great against just about every trainer you are likely to come across, including Leon, and it's great speed also helps considering how slow the Grookey line is. Gastly's movepool is also quite deep, as it can learn the likes of Energy Ball, Thunderbolt, and Dazzling Gleam alongside its Ghost and Poison-type attacks.
5 Avoid: Duskull
While the Duskull line is great for Trick Room teams and has the added bonus of being able to utilize Evolite very effectively, it just isn't as viable an option for regular playthroughs. It's niche usage aside, it also has a much more shallow movepool than the Gastly line that doesn't help to cover either Grookey's weaknesses nor any other team member's weaknesses.
4 Catch: Bronzor
Although Bronzor and, by extension, Bronzong, is more of a support Pokémon, it is still very useful when paired with Grookey. Its Psychic-type covers Grookey's Poison-type weakness while also being immune to it due to its Steel-typing, on top of having either Heat Proof or Levitate to cover one of its own weaknesses.
If The Crown Tundra is available to you, then Metagross would technically be the better alternative, but it has the downside of being very slow to train and not evolving until much later in the game.
3 Avoid: Galarian Ponyta
Although a Psychic-type Pokémon is a great companion to Grookey and its evolutions, Galarian Ponyta isn't a Pokémon that should be used alongside it. Aside from not having a very diverse movepool, even after evolving into Rapidash, it is simply a weaker version of Gardevoir and Hatterene while sharing the same disadvantage of being both weak and strong to Poison-type Pokémon. On top of this, none of Ponyta or Rapidash's abilities are particularly useful in a standard playthrough, though Run Away can be helpful against strong overworld Pokémon that normally require a Pokédoll to run from.
2 Catch: Dreepy
Of all the Dragon-type Pokémon available that pairs well with Grookey, the Dreepy line, while being hard to find and catch, is probably the best option. This ridiculously fast pseudo legendary covers just about everything that Grookey is weak to due to its diverse movepool, as well as being a great fast Pokémon alongside Gastly despite sharing the Ghost-type. Dragapult is also one of the earlier pseudo legendary Pokémon to evolve, meaning you will most likely have one for both Raihan and Leon, where its Dragon-typing comes in very handy.
1 Avoid: Jangmo-O
Although it is also a pseudo legendary Pokémon alongside Dreepy, Jangmo-O definitely suffers due to its unfortunate typing. Aside from being four times weak to Fairy-type attacks, a type that comes up multiple times in the story on strong trainers, its role as a tanky attacker puts it in the same boat as Grookey's line, with both Kommo-O and Rillaboom having comparably stats across the board. Unless you are building the rest of your team to deal with common Fairy-type Pokémon, Jangmo-O is better left alone.