Where you wear it (under or over foundation, and for touch-ups) is not the issue . What matters: the concealer’s texture. It should be light, regardless of how much you pile on. This means it layers so well that you never get light patches on your face.
Who is good at layering? Popstar Cosmetics’ Screen Concealer ($25) and Cle de Peau Beaute’s Concealer SPF25 ($105).