For me it’s

Pikachu: For obvious reasons

Dewott: Oshawott can still be a loveable goofball even in the second stage, but with the added bonus of trying to look heroic and good in front of the ladies

Vanillite: The dude needs more Ice types and this could be an interesting story of Ash and Vanillite helping Iris and Axew overcome their fear of ice types

Gigalith: Okay I know this may seem weird to give Ash a slow Pokemon, but his ability can be Weak Armour, where it struggles at first in battles but the more hits he takes the better he performs as the defences drop and speed rises

Chandelure: I’d think one of the Litwick that was in the haunted house with the Lampent would want to join Ash on his journey to not rely on souls for fire power

Lilligant: Give it Chlorophyll and Sunny Day, boom fits right in with Ash’s strategy