I think Dracovish should be there as well since it is a Gen 8 Pokemon like Sirfetch'd.

Honestly, i am a fool. Dracovish is my favorite gen 8 mon and i totally forgot Ash already has one.

Just like others, i am going to give my reasonings for Ash owning them

Inteleon > Fits Ash the best, since he usually gets a starter that is really timid or has a backstory. As such, Sobble line fits him the best.
Rillaboom > Will stand out amongst the other Grass starters, as its shiny. Where Sobble is the timid one, Grookey is the rebellious one, since Ash found it in the wild Area, it being rejected by the Silverback Rillaboom for being a different color (Rillaboom as such has the backstory, which is also new since that hasn't happened with the Grass starter yet).
Sirfetch'd > Being a battle-hardened pokemon, he joins Ash after seeing its prowess, having tied with Pikachu. Early on, Farfetch'd takes the timid Sobble under it wing as its father-figure and is annoyed by Grookey constantly picking fights with Sobble.
Grimmsnarl > Caught in Glimwood Tangle. Its a mischevous Pokemon.
Mr. Rime > After Ash brought his Mr. Mime in for some training time with Grimmsnarl (To teach Grimmsnarl Light Screen), Mr. Rime watches from a distance. He eventually joins Ash and Mr. Mime and he and Mr. Mime develop a friendship. Mr. Rime proves to be a reliable battler, bringing Ash the win against Gordie, despite a type disadvantage.
Dracovish > Obtained after revived from a fossil. Becomes a reliable and stronger partner and develops a friendship with Rillaboom and Grimmsnarl. Eventually send to Oaks lab, so Oak and Tracey can observe it, as Ash's Dracovish is the first revived Dracovish fossil in the modern world.
Copperajah > A Gigantic Copperajah goes haywire in the Wild Area (Which has a couple of villages and farms). Ash and his companions (Sonia and Gloria) are going to help them. Ash uses Inteleon, Rillaboom and Sirfetch'd to calm down the Copperajah, which shrinks in size. They discover its wounded and bring it to Nurse Joy and after its brought back to full health, it decides to join Ash. Becomes Ash's Gigantamax Pokemon and second strongest Galar Pokemon behind Sirfetch'd
Dragapult - Ash catches it after having battled Dreepy. Becomes a reliable teammember.

Sirfetch'd, Inteleon and Rillaboom would battle the most out of the team, but there isn't a truly dedicated ace.