First of all, it’s objectively a beautiful double page close-up of the character who before this flashback was the most mysterious yet important in the series and we had all been waiting for years to know more about him.
The details are just great and seeing Roger smile like that makes me want to smile, the same way I smile when Luffy is shown truly happy.
The second aspect is the psycological one. No one before could have predicted that upon discovering the One Piece the King of Pirates laughed from the bottom of his heart.
The sheer surprise of this reveal makes it memorable, and it makes you question even more: what could the One Piece possibly be to make Roger and his crew laugh like that? And also: what kind of emotions does a man who only has very little left to live feel once he finally achieves the dream he’s been pursuing his whole life?
And last, perhaps the most important aspect: the end of One Piece. Everything about this flashback, culminating with this chapter, screams “the end of One Piece is near”, this page in particular.
Just in front of Roger there is the One Piece, the goal we’ve been looking forward to for many years, it’s almost like if the page was just a few centimeters longer to the left we could have actually found out the truth now. In a way, the emotions Roger is feeling in that panel are the same we will feel once the journey ends. Honestly realizing how close we are to the end left me speechless and I still have many mixed emotions inside me. But just like Roger’s, our voyage eventually has to end as well.