What do you think is the best alternative to fossil fuels?

A mixture: geothermal in some areas; wind, tide and/or solar in others. There is no one answer fits all. However; the good news here is that independent nodes on a network provide more security and stability than a system like our current electrical grid where an outage in one spot affects multiple states. P.S. Bio Diesel has net negative environmental effects.

I like biodiesel. It can be made from soy beans, or algae, or other sources. It is cleaner burning than fossil fuel diesel.

Dilithium crystals from Vulcan.

Probably if they come up with clean nuclear energy then I think that it would be the most powerful. But I read that even if they do come up with clean nuclear energy, all of the cement and building materials in order to make it are really bad for the environment so idk.