Soy milk is the only common plant-based milk with a protein content comparable to dairy milk, which has a protein content of up to 3.4g per 100ml (the lowest fat cow's milk sold has the highest proportion of protein). Nic also found that pea milk has a similarly high protein content, around 3.3g per 100ml, but this may not be as widely available as other plant milks.
Here is a table based on data from various sources (because products vary), partly on the tables on One Green Planet and sources as linked in the table.
1 Other nut milks have comparable content.
Does that make them a bad substitute for cow's milk?
In terms of protein, you could say that about most plant milks have significantly less protein than cow's milk, but you can easily get plenty of protein from other sources.
Plant milks are also low in calcium and B2 unless they have been fortified (which is common).
Plant milks do provide various vitamins, minerals and beneficial fats (not saturates like cow's milk) and quantities of fibre, and have different flavours, textures and culinary properties which make them suitable for various purposes (other than providing protein)!