Alice in wonderland oil. Much of what makes a decent lubricant, also makes it attract and adhere dirt. Even the cleanest lubricants need at least a dry wipe before re-applying. There's also the effect of wear by product (steel dust) which accumulates within the chain, so there's no magic answer.

As for biodegradable oil there are a few, and they're OK, but in my experience the fact that chains are exposed to weather might make them problematic because of things like rancidity, or low resistance to water wash out. Dry biodegradable lubes are tricky, since most so-called dry lubes are teflon or something comparable and a volatile solvent, and other than water there aren't many green options.

I expect that we'll see more choices, and greener lubes every year, but in the meantime, other than the solvents we put into the air, the environmental impact of the few drops of oils used on bike chains is miniscule.

In fact, it might not make sense to use a "greener" lube if it shortens chain life, since the environmental impact of making a replacement chain and disposing of the old one sooner than necessary could be far more material.