The most eco-friendly toothbrush is one that can be fully composted.
If you're looking for a truly eco-friendly toothbrush, you'll want to shop for options made with plant-based materials, such as bamboo.
You may also want to consider an electric or rechargeable toothbrush. Rather than throwing away a standard plastic or plant-based toothbrush every month or so, with an electric toothbrush, you can simply replace the head.