There are good reasons to say that PAM Dirac is indeed the greatest physicist.
He was present at the birth of quantum theory and took part in the debates as to the merits of the wave theory formulation due to Schroedinger, and the matrix formulation due to Heisenberg. Dirac managed to show that they were two aspects of the same thing. This alone would make him a giant. It is not clear to me that there is any similar achievement elsewhere in physics.
Like Einstein he went seeking for a new theory, and managed to produce it by himself in under two years, resulting in the Dirac equation that some think is the most beautiful equation there is. Although Dirac did not realise at the time, this theory predicted the positron and so antimatter. This prediction alone would make him a giant. It is comparable to the bending of light prediction of General Relativity.
The Dirac equation also accounted for the fine structure of the hydrogen atom, ranking as one of the greatest explanations of the unknown in physics, comparable to General Relativity explaining the orbit of Mercury.
As a piece of mathematical physics, Dirac’s work it is as brilliant as Goedel’s incompleteness theorem and as unexpected in many ways. Einstein’s General Relativity and Maxwell’s equations are the only comparable achievements, but Dirac’s use of arcane 19th century mathematics is astounding. Einstein needed Minkowski but Dirac did it all by himself working day and night in a small room in Cambridge. It is arguably a greater achievement that the annus mirabilis of Newton in 1666, also in Cambridge.
Einstein spent the rest of his life after producing GR looking for a unification of GR and QM, but never went beyond Dirac’s unification of SR and QM. Dirac’s work directly informed all that came after, especially the move of quantum theory from quantum mechanics to quantum field theory.
Finally, Dirac invented a notation for describing quantum systems, the Bra-Ket notation, that is still in use today both for teaching the theory and for using it. The only notation that can compare, that I know of, are Feynman diagrams. This alone would also make him a giant of the field.
So in terms of his lasting contributions unification, explanation, prediction, and simplification, I would indeed vote for Dirac as the greatest physicist.
source: quoran John Schlesinger