One of the few villains with Teach to actually bring consequences in the series. Even an arc involving two emperors couldn't do this. If you're not a flashback character you're usually nigh unkilleable in this series.
Akainu killing the fan-favorite, MC's brother in front of his and readers powerless eyes after 60 chapters spent on rescuing him and mindbreaking said MC will always make Akainu stand out compared to other villains.
Him being an authoritarian hardass makes it a perfect foil to Luffy free spirited goofiness. Heck, he's the only relevant authoritarian hardass in the series, making him also stand out from chaotic pirates who wants to start a big war/topple the WG/change the world by plunging it into chaos.
While I don't think villains need to be super complex to be entertaining, the drawing of a child Akainu in a SBS could imply he has rough past that shaped his beliefs and he's not just LE bad because.. he's LE bad just because
Sparing Aokiji when their fight was supposed to be a fight to death is also something I find interesting.
And last, some of the most common complaints about the series are the lack of tension, fake deaths and consequences. He brought them for the time of an arc.