In your opinion, what is the most beautiful math equation ever derived?

this is such a good question that i feel like i have a really insightful answer but i cannot pull out the MOST beautiful one.

in my calc class a few days ago i learned about integreation by parts, though i think khan academy explained it way better than my class, by deriving the forumal off the product rule, which was really interesting to see.

though i feel like there's probably some more interesting equations that are even more beautiful, but i don't really remember it.