In these months at school I am studying Heat engines, Carnot cycle, Nuclear reactors, and the efficiency of a generator, so I applied this with Industrialcraft!
This is my table of the income (revenue), cost (sum of all the valuable ores (without cobble,wood,rubber) ),and the efficiency!
efficiency = income/expenditure
(I will update this link!)
EDIT: Income of geothermal: 20EU/t but the efficiency is correct!(I updated the link)
So the best choice is the windmill! Cheap without a nuclear meltdown and infinite power!