So I have read that sugar (cane) is way more efficient than corn, (and that Brazil leads the way in taking initiative to actually bother with this advance in biofuels sustainability), and that, based on multiple independent sources, they produce EIGHT times the energy than what they put in to produce it.
But could the little-mentioned hemp, (via any of its relevant ethanol production methods), be even better than sugar? Or could something else (let's limit it to a small scale if necessary) actually be the most sustainable feedstock for ethanol?
Assume there is no licence to grow particular crops like industrial hemp. (In your respective country, there may, but I want to take out money from the equation and the sustainability impact of that licensing factor per se.)
Assume one has no legal requirement to denature the ethanol (as in many countries this is the case), or filter it with 15% minimum regular gasoline (like in the USA). This is pure ethanol, only including and requiring what is needed to make 100% pure, and efficient, automobile burning fuel.
Assume scalability issues are not there. Maybe this is for the purposes of small-scale, personal production, where time isn't an issue, except when comparing time per energy output between different ethanol feedstocks - so economic viability needs not matter, except where the difference affects even small-scale sustainability comparison.
Also assume that all feedstocks can be grown effectively in your area. (So that they can be compared on an equal playing field.) You have a patch of suitable land, and are ready to grow any of them, requiring whatever inputs they respectively need, in the most sustainable way possible.