The conventional approach to selecting suitable areas for wave energy development is based primarily on mean wave energy. However, despite its importance, this single metric is not sufficient and may lead developers to the wrong areas. This work introduces a new index, the Wave Exploitability Index (WEI), for the selection of optimal locations for wave energy harvesting. This is complemented by a new classification of the wave energy resource, which distinguishes among five different classes, from Class I to V, from lowest to highest wave power. The combination of the Wave Exploitability Index and the wave resource classification constitutes a novel approach to selecting potential areas for wave energy development, which is applied in a global scale study. We find that Classes IV and V (mean wave power over 40 kWm-1), characteristic of upper latitudes, typically do not present high WEI values. At the other extreme, Class I (mean wave power below 10 kWm-1) is characteristic of enclosed and semi-enclosed seas (e.g., Mediterranean Sea, Gulf of Mexico, Black Sea) and presents the lowest WEI values, with little potential for wave energy development. The highest WEI values coincide with Classes II and III, characteristic of tropical and lower-middle latitudes, which present a relatively high mean wave power (10-40 kWm-1). We find that some of the areas belonging to Classes II and III are of great interest for wave energy exploitation despite not having received a great deal of attention (e.g., SW Australia, Chile).