Personally, I believe Akainu is the worst so far. Villains like Arlong, Moria, and Blackbeard are bad, to be sure, and they've all done acts that can only be described as unforgivable, but they don't try to hide their villainy. They are fully aware of the fact that what they do is wrong, and they openly acknowledge that. It's bad, but unlike some of the others, they don't sugarcoat their heinous deeds.
Crocodile and Eneru are a bit worse, as they do evil while pretending to be decent, honest people. They were given positions where people trusted them, and they abused that trust by ruining the lives of those people through manipulation. But Crocodile, unlike the other villains, does seem to have a decent side to him, and Eneru was little more than a maniac who was driven to madness by the power given to him. He's powerful, but take that power away and he's harmless.
Spandam and the Tenryuubito are even worse still, by doing the most underhanded, pathetically cruel acts imaginable, and totally getting away with it through means of their lofty titles. But, like Eneru, what happens when you take that power away? They're the kind of villains that the audiences love to hate, but in the end, they're nothing other than an example of undeserving people who just happened to have positions of authority thrust upon them.
However, Akainu is a completely different story, as he has the worst qualities out of all those villains. The kind of person who uses trickery to achieve his goals, attacks defenseless people, manipulates people in the most heinous ways, and even murders defenseless people. All things that the other aforementioned baddies have done, but Akainu is different, because unlike the rest, he genuinely believes he's justified. He does have a fancy title, like Spandam, but he earned that title. He has used his methods to attain the position of Admiral, and that makes him all the more despicable, as nobody can argue with that. He openly does evil, he lies to his victims to get them to do what he wants, he has scores of men at his command, he's both strong and experienced, and he doesn't see anything wrong with what he does. In other words, a true scoundrel, through and through.