What are you using it for? I use almond milk for my cereal,etc because I like it better...... but dairy for gravy and sauces or anything savory, as I find substitutes too sweet,even unsweetened versions. If you have to use a dairy sub, (ie lactose intolerant) then just go with the cheapest.
If you're looking for "lowest" in categories, then you could just use water much of the time. But as TAMMYOKAHARA mentioned, unsweetened almond and coconut milk are where it's at. The unsweetened soy milks are higher (rice milk also tends to be higher in the sugar department, and oat and hemp milks tend to be expensive unless you make them yourself), but still less than skim milk. But you have to ask, why are you substituting? Soy milk, 'regular' or 'light', will give you good protein (up to 7g per cup), whereas you'll get at most 1 or 2g from the other alternatives. If you're using the non-dairy milk in baking and to keep things moist, you need *some* fat, so the lowest calorie versions won't work well.
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