Of course, pi isn’t really an equation, but as the universal constant it deserves priority of place, much like “the” does as the definite article in English. This is my shortlist of knowledge that cracked open the doors of discovery and let man step forward into the future. These also may be Booker Prize shortlist titles next year, with any luck.
The Universal Constant π
The Pythagorean Theorem a2 + b2 = c2
The Law of Universal Gravitation F = GMm / r2
Newton’s Second Law of Motion F = ma
The Law of Special Relativity E=mc2