I believe Simon Stevin is the most remarkable alumnus of Leiden University. He enrolled at the university in 1583. In Leiden, Stevin published books on mathematics and physics, and he also became known for his valuable contributions to hydraulic engineering and land surveying. His importance is highlighted by this quote from Istvan Szabo: "Stevin was probably the first scholar to combine theory and practice in the most effective way and therefore rightly bore the title 'engineer'."