Chaos Black Spray | Warhammer: Citadel Tools

When in doubt, you can't go wrong with this product.
The good thing about black is that it darkens hard-to-reach areas where a brush can't fit, so you can get away with some imperfections even if you don't paint perfectly.

Black undercoat spray (primer).
Games Workshop sprays contain primer, which helps prevent the paint applied over it from peeling off.
Be aware that cheaper primers may not contain primer.