Everyone should have a M.A.C #130 Short Duo Fibre Brush ($40). This tiny stippler gives perfectly executed makeup no matter what you use it for. The perfect blend of natural and synthetic fibers make it ideal for use with all textures. It blends perfectly, shapes precisely, gives an airbrushed finish, and easily transitions from one medium to the next. Here is a list of things I use the #130 for:
*Serum
*Moisturizer
*Primer
*Foundation
*Powder
*Blush
*Highlight
*Contour
*Blending

You can see how many uses I get out of the M.A.C. #130 Brush in my latest tutorial:

Monochromatic Orange Tutorial (Full Face Using 2 Brushes) from Marissa Rhoades on Vimeo.