The very best and purest are green foods. Wheat grass (you can get it in powder form to mix with water), hemp powder is one of the best, also green veggies. My husband is vegan, eats no meat, dairy, processed foods, eggs, etc. and has had his protein tested several times...results are higher protein levels than when he was a meat eater. I am also vegan, but haven't had my levels tested...my energy level is amazing though, and it wasn't always this way.
these are good sources of protein when you're vegan, but if you'e not i think it's easier to get it from meat and dairy rather than powders! i'm vegetarian, not too concerned about how much protein i consume, but i think i get enough from milk, cheese, beans and things like that.
Here's an interesting blurb from the University of Illinois:
As you seem to know, most plant foods do not have a "complete" protein, that is, they do not have the right proportions of all the essential amino acids that people require. By eating a variety of plant foods over the course of a day, you can get enough of all the essential amino acids that your body needs because the amino acid pattern of grains, for example, 'complements' the amino acid pattern of legumes (dried beans). Seeds and legumes are another complementary pair.
The amino acid pattern of soy is different from most plant foods in that it is a "complete" protein all by itself. Older studies based on a rat's protein needs reported that soy was an 'incomplete' protein, but rats need about 1.5 times as much methionine as humans. Newer ratings based on human protein needs rate soy protein very high. In fact, the PDCAAS (protein digestibility-corrected amino acid score) rating, which is used by the World Health Organization and the Food and Drug Administration, gives isolated soy protein its highest rating -- 1.0. That is as high as egg white protein and the milk protein casein, and higher than beef protein.
This is good news for me since I eat only TOFU with a sprinkling of soy and Szechuan sauces at least 2 lunches a week.