That is mostly down to what you're painting. I think mostly you should get black, white, primary colors, and then a foundational shade of any color which you will use in large amounts.

A black and a white paint are both very useful. I don't particularly like the Vallejo white, preferring the Pro-Acryl or Two Thin Coats whites.

Primary colors; red, blue, and yellow are good ideas because while I wouldn't recommend fully mixing all your own colors, having these colors (along with white) will allow you to mix into most other colors to create highlight shades. I'd recommend bright, saturated shades.

I personally get a lot of use out of VMC Chocolate Brown, Pale grey Blue, and Hull Red. However a lot of that is down to what I'm painting.