This is a debate that has been ongoing for eons of years through the lens of philosophers and scientists as well as amongst friendly gatherings.
The scientific answer leans towards the egg where evolutionary biology suggests that species evolve through small, gradual changes, meaning that at some point, a bird species that was not quite a chicken yet, laid an egg, and due to genetic mutations or variations, the egg hatched into what we would recognize as the first true chicken. This process is a part of evolution, where different species emerge over millions of years.
Eggs have been known to be laid by birds and other reptiles long before chickens existed, meaning, “Egg” as a concept predates chickens.