Oat has quite a pleasant taste, works well in coffee plus a much smaller environmental impact seems like the logical choice.
Not Milk is my favorite, followed closely by soy. Oat tastes good but has a thin mouth feel.
I feel it, I drink milk seldomly but oat works when I crave some cereal or want to lighten up my coffee every now and again. Soy is a solid choice though as well, pretty even with oat when it comes to global impact!
One could consider to stop drinking coffee. Coffee itself has a rather large environmental impact as it is. Or drink it without milk-like substances.
For sure, my coffee stays black 95%+ of the time but change is the spice of life. Nonetheless, you highlight a solid point regarding the consumption of coffee. Being soo prominent world wide must make it detrimental environmentally.