Akainu has demonstrated Intelligence, initiative and resolve. His actions were in the best interest of the marines' objectives. In fact, he was the only one at Marineford who did his job properly. Sengoku even let Luffy free Ace from under his nose, while Kizaru and Aokiji were quite useless in the grand scheme of things. Wasn't the point of the whole point of the battle to make sure that Ace was executed and Whitebeard defeated?
Additionally, he is also the strongest of the 3 admirals. In such an era of instability where pirates and dangerous criminals are running rampant, I would say Akainu is technically the better candidate. He ruthlessness is actually a pro rather than a con. At the same time, he doesnt seem like the guy who would start a fight with someone unless he knows he can win.
Also, lets not forget that the marines exist solely for the sake of the World Government (who dont really have noble intentions, as we have seen). Akainu makes more sense as a Fleet Admiral.