As someone thats gone biofuel before. I have to say wheat or sugercane. Both have a low return but can be automated in such a way that you need zero input to them. At that point you can make up for the lost of efficiency with just bigger farms and extra fermenter. If you have Thaumcarft you can automate the wheat farm with a few golems bypassing the need for really anything.
As an extension to this, you can use the lilypad of fertility if you have access to it to make the farm grow even faster.