That will depend on who you're asking, as opinion differ from artist to artist, and from art teacher to art teacher.
For instance, in Oil Painting for the Beginner i suggest the following basic list of colors as a starter:
· White Lead (Flake White)
· Prussian Blue
· Ultramarine Blu
· Viridian Green (Chro-moxide Green
· Transparent)
· Yellow Ochre
· Cadmium Yellow
· Venetian Red
· Cadmium Red
· Alizarin Crimso
· Burnt Sienna
· Burnt Umber and
· Ivory Black