Tl:dr, all of them.
Sanding is always a multi-stage process. Coarser grits remove more material but leave a rough finish, finer grits remove less but the finish is cleaner. For proper use you always want to move coarse to fine, as the finer grits will clean up after the coarse ones before it.
For molded plastics I’d say start at 120grit and see how that does then move up from there, probably max out around 1k depending on how the finish on the rest of the area is.