The best bean is variety
The main concerns with bean are that many are incomplete proteins: they are relatively low in Lysine and Methionine.
However, there are some exceptions. Most notably Soy is a complete protein with high levels of Methionine so Soymilk, Tofu, Edamame, and Natto are all good protein sources. Similarly, Chickpeas (garbanzo) are higher in Lysine than most beans, while not complete for essential amino acids there easier to complement. Not strictly beans but Quinoa, Buckwheat, and Quorn are all complete protein sources too.
Another consideration is that Methionine and Lysine are plentiful in other foods and easy to complement. Eating beans with Rice, Wheat, or Couscous would get you your full set of amino acids in one meal. Note that Corn is also Lysine deficient which is a problem in some diets.
The best approach to complete proteins is variety, then you ensure you get a variety of other nutrients (vitamins, minerals), and get to eat interesting things instead of the same thing everyday. Soy has become the de facto vegetarian protein substitute for good reasons but a health diet is a balanced one and there's no reason not to give other beans a try.