Tide brand detergent is the current market leader, with various types of HE and standard detergents, and Tide 2X Ultra for HE washers receives high marks from Consumer Reports. The "2X" refers to the fact that the detergent is concentrated, which means it can clean more loads with less detergent (and it's stored in a smaller, more compact bottle). It also includes a color-safe bleach alternative, so you don't have to use separate bleach. This detergent comes in a liquid form only and costs about $0.24 per load.
Because an HE washing machine tumbles instead of agitates, it creates more suds than a traditional machine. Therefore, HE detergent must be low-sudsing.