Hans Bethe

One of the truly great physicsist of the 20th century, Bethe did fundamental work in most major branches of physics, including atomic, nuclear, solid state, high energy and astrophysics. One might expect that his selected works would be fairly inaccessable, but surprisingly this is not always the case since much of his work (but not all, since some was previously classified or unpublished and some was previously not translated from the original German) has found its way into the textbooks and so might be somewhat familiar.

Bethe was put in charge of the theoretical division at Los Alamos during the atomic bomb project by Oppenheimer, and had a singularly unique insight on what is required to design an atomic weapon. Therefore, his commentary on the famous case of the A-bomb kid (see here and here), while not as well known as the case itself, is authoritative:

"I have seen the paper of the Princeton student, and to the professional it is not impressive. He presents with great seriousness a calculation of the energy yield of a bomb, assuming a certain amount of fissile material, and assuming the efficiency of the nuclear reaction - a grade-school exercise in multiplication - but he never gives any argument for assuming this particular efficiency, which in fact is much too high for an amateur bomb. He mentions a few types of explosive that could be used to assemble the fissile material, but he does not discuss the intricate shapes in which the explosives has to be cast. To cast the correct shapes, and to prove experimentally that they were indeed correct, took us months at Los Alamos, with hundreds of technicians, and this is just one of the many problems that are not discussed in the open literature." cf. Hans Bethe: Prophet of Energy by Jeremy Bernstein for an excellent and even more detailed discussion

Here is a video of Bethe giving a lecture on quantum mechanics.