My heart still skips when I think about that scene in 'One Piece' — the moment everyone talks about. If you want the exact spot in the anime where Ace actually dies, the pivotal episode is episode 483. That episode gives you the immediate, devastating moment when Ace is fatally struck and Luffy holds him; it's edited in a way that really lands emotionally because of the music and the reaction shots.
If you want the full emotional arc around it, don't stop there. I usually recommend watching the build-up from roughly episode 477 onward through 483 so you get the rescue attempt, the fights, and the stakes. After 483 there are a few episodes that cover the immediate aftermath and Luffy's breakdown, so watching 484–486 (and even the next handful of episodes) helps the scene breathe and lets the grief land. For context, the whole Marineford sequence stretches across many episodes, so if you have the time, give the whole arc a watch — it’s brutal but powerful. I always make sure I have tissues handy and a quiet evening when I rewatch these scenes.