It's impossible to definitively say which scientific finding is "the greatest." It depends heavily on what criteria you value most. However, here are some contenders for the title, each with its own impact on our understanding of the universe:

1. The Theory of Evolution by Natural Selection: This theory, proposed by Charles Darwin, revolutionized our understanding of life on Earth. It explains how organisms change over time, adapt to their environment, and diversify into new species. It's a fundamental pillar of biology and has had a profound impact on medicine, agriculture, and our understanding of our own place in the world.

2. The Laws of Motion and Universal Gravitation: Isaac Newton's laws of motion and gravity laid the foundation for classical mechanics and allowed us to understand and predict the motion of objects on Earth and in space. These laws are essential for engineering, astronomy, and many other fields.

3. The Periodic Table of Elements: Dmitri Mendeleev's periodic table organized the elements based on their atomic properties, revealing the underlying order of the universe and predicting the existence of undiscovered elements. It's an essential tool for chemists and scientists across many fields.

4. The Theory of Relativity: Albert Einstein's theory of relativity (both special and general) changed our understanding of space, time, gravity, and the universe. It revolutionized physics and led to breakthroughs in cosmology, nuclear physics, and GPS technology.

5. The Structure of DNA: The discovery of the double helix structure of DNA by James Watson and Francis Crick in 1953 was a landmark event in biology. It unlocked the secrets of heredity and revolutionized our understanding of life, leading to breakthroughs in genetics, medicine, and biotechnology.

6. The Standard Model of Particle Physics: This model describes all known fundamental particles and forces in the universe, except gravity. It's a triumph of theoretical physics and has helped us understand the universe at its most fundamental level.

7. The Big Bang Theory: This theory explains the origin and evolution of the universe. It's supported by a vast amount of observational evidence and has helped us understand the age, size, and composition of the universe.

It's crucial to remember that these are just a few examples. There are countless other important scientific discoveries that have transformed our understanding of the world. Ultimately, what constitutes the "greatest" finding is subjective and depends on individual perspectives and values.