Human milk is low in protein, high in lactose and has a very low ratio of casein (the curd) to whey. Closest is probably horse (mare), but human is significantly higher in fat so probably tastes better.

Fun fact: humans are the only species that drinks milk directly from other species. We're also the only species which drinks milk past infancy. Don't worry, though, it's "normal, natural and necessary".

The fact that we tend to lose the ability to digest milk past infancy suggests that this isn't but is rather something that we have relatively recently started to do (perhaps less than 10,000 years ago, apparently and in Europe) and are still adapting to it, which is quite interesting.