LED (Light Emitting Diode) and Ceramic Metal Halide (AKA Light Emitting Ceramic) are going to be your most efficient.
But most will agree, it is really hard to get the large dense colas with anything less than a decent LED or a decent HID (High Intensity Discharge light, regular Metal Halide, High Pressure Sodium and the new CMH/LEC are all in the category of HID lights).
HPS would be the next most efficient in the amount of electricity used versus the amount of use-able light provided to the plants (this is known as PAR, or photo-synthetically active radiation).
High Output T5 fluorescent lights and some CFLs can be used for flowering with decent results if scrogging is used heavily. These and the cheaper LED units, like many marshydro and many others are at the bottom of the list of efficiency, but they still blow away the older T8 and T12 type fluorescent lighting.