Honestly, I see a lot of the same three moments pop up over and over. The most brutal, obviously, is the panel from the manga where Akainu's fist is just... going straight through Ace's chest. Fan artists love to zoom in on that impact, the shock on Ace's face, the absolute horror in Luffy's eyes right behind him. It's graphic, but it captures the sheer sudden violence of it.
Another one I keep seeing is the aftermath, where Ace is already collapsed and Luffy is just holding him, that blank, shattered look on Luffy's face. Artists will often play with the lighting here—making the Marineford battlefield all dark and gloomy but having a beam of light just on the two brothers, or focusing on the 'Will of D' necklace in the debris. It's less about the gore and more about the emotional wreckage.
Then there's the symbolic pieces. Those are my favorite. I saw one where Ace's body is dissolving into swirling flames and phoenix feathers, but they're being snuffed out by black, volcanic rock. It ties back to the logia element disadvantage, which adds a layer of tragic inevitability to the whole thing. The fan art that digs into the devil fruit symbolism always feels smarter to me than just the straight depiction of the punch.
I guess the final scene people latch onto is Ace's last smile. That panel gets recreated constantly, often with a transparent or fading effect, like he's already becoming a memory. Sometimes they'll put the vivre card ash floating around him. It’s less about Akainu and more about Ace's character exit, which, given how much fans loved him, makes sense why it’s so popular.