It's hard to pinpoint exactly when, but One Piece has many amazing moments and arcs. Maybe it gets especially great when the Straw Hat crew starts facing more powerful enemies and uncovering deeper mysteries of the world.
Well, One Piece has been consistently good throughout, but many would argue that it reaches a peak in terms of story depth and character development during the Whole Cake Island Arc and onwards.
It's subjective. Some people think it gets really good from the beginning, while others might say it starts shining around a certain arc like the Marineford Arc.