So I already know a lot of reactions will be “wtf” but lemme cook.
Itachi has a lot going against him and for him as a better brother. His relationship with Sasuke is complex and deep, it’s hard to summarize it and do it Justice, but I’ll try. On the other end, Ace as a brother figure is maybe slightly above average? I think it mostly comes down to whether or not Itachi’s failings drags him down to Ace’s level or below.

Itachi did a lot of wrongs in his life, he greatly hurt Sasuke both physically and emotionally, he killed people and ultimately let himself be seen as a villain. But people don’t seem to realize he did everything with the best interest for Sasuke at heart.
Itachi hated violence and feared the outbreak of another war. Backed into a corner, Itachi consigned himself to destroy the Uchiha clan to preserve the peace of the world.
But more importantly, it allowed him to save Sasuke’s life. And on top of that, Itachi threatened Danzo that, if Sasuke were injured or harmed, Itachi would spill secrets to other nations, risking war.
Itachi loved his brother so much, he’d risk war for his safety—something he was unable to do for the clan.
This doesn’t excuse his actions but it shows how he’s always putting Sasuke first, keeping him safe even when Sasuke isn’t aware of this. This happens later on when Itachi removes Orochimaru from Sasuke’s body and implants a one usage of Amaterasu in Sasuke’s eye to defend him from “Madara.”

Itachi planned everything including his death out to make Sasuke a hero and make him untouchable by the shadowy elements of the village.

On the flip side, you have Ace. He isn’t as involved in Luffy’s adventures and both siblings are really independent. They have faith in one another to carry through the day. But you do see glimpses of Ace caring about Luffy.
In Impel Down & Marineford, Ace is saddened at the news that Luffy is risking his life for Ace’s.
During the flashback post-Marineford, you see Ace defending Luffy against some small time thugs and pirates.
You see Ace getting defensive when telling Yamato Luffy’s dream and he’s so proud of Luffy, he shows both Whitebeard & Jinbe his first bounty.
There’s a deep level of love that that, while not as explicitly explored as Itachi’s love for Sasuke, can always be felt regardless.
It’s a simpler relationship that some might find more appealing.
So who do you think was a better brother, given their circumstances? The genius of the Uchiha Clan or Whitebeard’s second division commander?
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