For me, as a researcher in Physics, Leiden is about Heike Kamerlingh Onnes' discovery of superconductivity. Superconductivity is, in my opinion, one of the most fundamentally intriguing phenomena in Physics. It has been relevant for over a century and has led to new fundamental and technological revolutions ranging from MRI machines to quantum computers.
During my 1.5 year stay in Leiden, however, I have come to realize that Leiden as well as Dutch people are not that aware of Kamerlingh Onnes. He deserves a statue in the city.