This post from Our World in Data sets out the environmental impacts (land use, greenhouse gas emissions, freshwater use and eutrophication) of dairy milk and several alternatives (oat, soy, almond and rice milk). It concludes that dairy milk has higher environmental impacts than all of the alternatives across all categories considered, and that there is no clear winner between the dairy alternatives.
Read more here. See also the TABLE explainer Environmental impacts of food: an introduction to LCA. A detailed analysis of dairy and plant-based milks can also be found in the report The role of dairy and plant based dairy alternatives in sustainable diets (PDF link), co-authored by TABLE director Tara Garnett.