Plant-based milks are now mainstream. Any plant-based milk is better for the environment than dairy. However, some are better than others.
Nut milks require smaller land areas and the trees absorb carbon while they are alive. Almond milk was one of the more popular choices but its production has been linked with billions of bee deaths. Almonds also require a huge amount of water to grow.
Legumes such as pea and soy and more water efficient and help fix nitrogen in the soil. There is concern about the huge amount of land that is cleared for soy production with the majority of soybeans are fed to livestock.
Hemp although not widely grown is one to watch as the whole plant can be used, the seeds for oil and milk, the plant itself for textiles. I do make hemp milk but find it has quite a strong flavour so I tend to mix it with other things.
Oat and rice milks are also a good environmental choices, although there have been concerns that rice milk contains inorganic arsenic.
Making your own milk
When I first started making plant milks I made soy milk so that I could make tofu. We bought cartons of plant milks. Plant milk especially organic can be expensive. Making your own milk is so much cheaper and you know exactly the source and quality of your ingredients.
At first I would measure everything and would strain the milk from the pulp. I then had the dilemma of what to do with the pulp and after a while of making biscuits and brownies etc it became a stress to keep up with making use of it. I then bought a high speed blender and now I don't strain the milk (except for soy which is a different process).
Homemade plant milk does separate in the fridge and isn't brilliant in hot drinks but for overnight oats unstrained milk is fine. It is cheap and easy to make. With a high speed blender you don't need to soak the nuts or seeds which is great if you have forgotten or you decide to make something that needs milk.
Oat milk is the hardest to make as the texture can be slimey but added to other nuts or seeds it can really enhance the creaminess of the milk.
There are lots of recipes online that you can follow but if you have a high speed blender like a Vitamix and you don't mind a bit of grittiness you can just throw a mixture of seeds and or nuts and water straight into the blender.
Dates - to sweeten
Vanilla essence or paste