I was puzzled by the concept of the ‘greatest’ and how it might be ascertained! Even in boxing, it is difficult to compare champions from different eras! Anyway, my thought in the area of physics is that the discovery of quantum mechanics represents one of the most beguiling, strange, mysterious and yet useful discoveries ever made.
Of course, no single person made the breakthrough, although perhaps Heisenberg might be credited. Quantum mechanics is practically useful in all manner of areas from neutron stars to electronics, and yet it is safe to say, with Feynman and Bohr, that no one who has studied it truly understands it. It represents a challenge to our conception of nature and reality. In this dual sense it is a good candidate for one of the greatest innovations of the last 100 years- essential to underpinning our understanding of the modern world and yet a challenge to its fundamental concepts.