Behr Ultra Pure White Paint.
This is my favorite paint to date.  One coat is all that I have ever needed on walls, trims, etc.  And, its brightness is amazing.  My walls look clean and bright.  Where I have colored walls, I use it for the ceiling and floor trims, which make the room look clean and fresh.

I primarily buy the Flat, but will pay the extra dollar for the Flat Enamel for surfaces that tend to get dirty faster and need clean up, like my kitchen cabinets.

Where: Home Depot
Cost: $24.95 / gallon