@maxterdexter:
My guess was that early series has way more punch and context to these stuff. The Mihawk moment is stronger because the promise for a bigger world was kept, The Kuro moment is stronger because Oda keeps doing these kind of things, The Bellemere moment was the first one of the long line of sacrifices the strawhats parents have done for them, and sets the divide of a troublesome past being confronted by hope and determination in the present, and so forth.
Later part of the series haven't been able to be fresh and new, as they are continuations of what came before, Rox and Imu can't fill the shoes of the warlords as impact just because they are bigger and better, Corazon might have hurt, and would have hurt more if it wasn't because he was standing atop a pile of dead mentors, "The children are watching" and can keep watching as long as they want but "A man dies when he is forgotten".
It is hard to keep the narrative strenght of the series for so long, and there is plenty left to explore, the recent new ground on creepyness of Big Mom's song and the terror/tragedy of her sixth birthday, Thriller bark even when made defliing bodies and keeping people prissioners on the shadows didn't reach these levels of creepy.
I agree that Imu (and probably Rox) reveal isn't as impactful as Mihawk's, but I think Kaido's is in that vicinity at least, as well as a couple characters from paradise, namely Raleigh, the admirals, and Whitebeard. Mihawk is the initial character who greatly expands the world of One Piece from one of light adventure to one where they're really having to get stronger and go to uncomfortable places to survive, but guys like Whitebeard and Rayleigh flesh the world out and fulfill distinct and specific roles.
Admirals are effective because we as an audience understand the ranks of the military and know they have to be there somewhere and when they do come, bam, they deliver as promised. Same with Rayleigh, a character who I wager most didn't ever expect to even meet demonstrates the power of the old guard and by way of likely parallel, is demonstrative of the power Roger might have if he were still alive in this day.
Now if Kaido had appeared similarly to Mihawk, without build up and as an entity occupying a space we weren't familiar with (as opposed to being an Emperor) then I could see how his reveal would be less impactful, but he meets and matches the hype in a chapter that I can visualize at just about any time from a creative standpoint. His design, personality, and motives are all interesting and as the last emperor revealed, he gets the, ahem, most epic entrance.
As for just the pure joy that comes from surprises, how quickly can we forget the absolute bewilderment that all fans of One Piece felt at the reveal of the giant hat. Oda's still got it man!
I'd agree with you and Robby about the flashbacks, though. Oda's really great at them, but those first couple up till Brook's are probably the cream of the crop, though I imagine that if the story was told in a different order any of them would be considered favorites. Here's hoping that Jinbe may be the recipient of another flashback that rivals the ones of old.