What is the most hardest equation

Some equations are famous for their difficulty or the profound challenges they present:

• The Navier-Stokes equations describe the motion of viscous fluid substances. Their existence and smoothness for general initial conditions is one of the Millennium Prize Problems, meaning a solution would earn a $1 million prize.
• The Riemann Hypothesis (related to the Riemann zeta function $\zeta(s) = \sum_{n=1}^{\infty} \frac{1}{n^s}$) is another Millennium Prize Problem. It's a conjecture about the distribution of the non-trivial zeros of the Riemann zeta function, and its proof would have far-reaching implications in number theory.

There isn't one single equation universally agreed upon as "the hardest."