The Grass-type starter, Rillaboom, is an absolute powerhouse. It has solid HP and Defense. Its Speed is not fantastic, but it can outspeed a decent amount of Pokemon that it is effective against.
The true strength comes from its high Attack stat. That means Physical Attack damage done by Rillaboom is incredible. The best moveset takes advantage of that and attacks with no mercy.
The best moveset for Rillaboom in Pokemon Sword and Shield
Rillaboom should have the ability Grassy Surge. That makes the move Grassy Glide attack with a +1 priority. Paired with a Same Type Attack Bonus and a Choice Band, this combination is one of the fastest ways in Sword and Shield to deal guaranteed damage. This is an incredibly effective way to start a battle.
Superpower will act as a very strong coverage move. This Fighting-type attack deals incredible damage, especially to Steel and Ice-types that may pose a threat to Rillaboom. This also allows a Gigantamax Rillaboom access to Max Knuckle, which further increases its Attack stat.
Knock Off is another coverage move dealing Physical Dark-type damage. This has the added benefit of removing the item held by the targetted Pokemon. It will be most useful against bulky opponents that may have Leftovers, Eviolite, or a Weakness Policy user before it gets triggered.
High Horsepower is the last attack in the moveset. It is yet another coverage attack. Ground-type moves can prove quite useful against opposing Fire-type Pokemon that may be hoping to knock Rillaboom out. It also provides Max Quake, which gives the team a boost to Special Defence when it hits. Rillaboom has plenty of moves to deal with any opposing Pokemon.