Goku going Super Saiyan has to be the more iconic moment just because One Piece is slept on in the west while Dragonball is known worldwide for laying the foundation for modern shonen.
One Piece is by far the better written series though. The foreshadowing, world building, and the sense of adventure are all GOAT tier, and One Piece tends to be a lot better about putting respect on the side characters. It’s one of the few long running shonen that never fell off.
(And it’s the bestselling manga of ALL TIME
)
It was fun growing up with DBZ and I’ll always respect it for what it started, but I get tired of cats claiming DBZ as the GOAT just because it’s the only anime they’ve watched. Diversify your bonds.