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As freebeard points out, a tandem rotor heli would be most efficient. Conventional helis waste about 15% of their power just counteracting the twisting motion the rotor imparts on the fuselage. Tandem rotors counter-rotate to offset the twisting.
Helicopters are inherently stable and require no electronics to fly. Having the blades above, lifting the chassis keeps everything upright. Quadcopters on the other hand are inherently very unstable, which is why they weren't popular until recently. They all require computer controlled gyro stabilization as they would just flip otherwise.
This is why I'm perplexed as to how GPS, accelerometers, sonar, etc made it to quadcopters well before helis. Quadcopters are mechanically simple though, requiring only 4 motors and no gearing. Helis require at least 1 motor, gearing, rotor head, and swashplate, and then there is still the tail rotor to consider.
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