Water by far. Sunlight is good only for clear days, and for only 6-8 hrs at the most. Plus it is too expensive to buy the solar panels for electricity. Under glass tubing is less expensive but still too short a day light time source. Wind is cheaper mechanically, and probably more hours of production overall. But still depends on sufficient air movement. Water from river flow is the most constant 24/7 production. Dams, are expensive but are multipurpose. Electricity, irrigation, drinking, recreation, flood control. Paddle wheels anchored along rivers or streams also produce continuous electricity and are cheap. They can be smaller or larger. Ocean waves and tides are being looked at as a continuous source also. All water sources of renewable energy are by far and away the most efficient. Water is heavy at 8+ lbs per gal. it does not take very much flow on a lever arm to multiply its effect dramatically.