I'd say the Admirals are villains in the context of the story, sort of. I would consider the Strawhats to be the "heroes," even though technically they're not and even state several times that they don't want to be heroes.
Crocodile looking as stoic (I'd say it's far more appropriately described as lazy) as he does is a good part of his character. He has no stated backstory, but it's hinted at in subtle ways. In Chapter 0/Strong World, he witnessed Roger's execution. One of the cover pages reveal that, due to Roger's execution, like many others, he once dreamed to become the Pirate King. It can be inferred from there that as he grew more powerful, he began to put more value in that power than trust (i.e. he trusted his power more than people)*, and eventually lost that initial fire in him and gave up the Pirate King dream - hence his lazy expression. He doesn't care as much anymore. Or at least, he's lost one particular fire in him After his plans at Alabasta fail, he loses what little will he had left in him, as evidenced by the cover page story where he decides to chill out in prison rather than escape with the Baroque Works members (bar Mr. 1, 2, 3), even though he was destined to go through Impel Down's torture. It only returned when he realized he had the chance to fight Whitebeard. He lost that chance, but ever since Marineford (we only see him right after it but whatever), he's looked more determined again, which is supported by the fact that he asks Daz Bones (Mr. 1) to go back to the New World with him.
*By putting those pieces together, you really get a new perspective on that one line of his back in Alabasta that initially sounds like a fancy way of telling Luffy "you're weak":
"Listen to me, brat! The more one comes to know of this sea, the less likely one will carelessly spout such boasts. As I said before, there's plenty of small-time rookies like you in the world. And the better you know about the level in this sea, the hazier such dreams become."
His own dream about becoming Pirate King became hazier. Most likely partially became he never grew strong enough, and partly because of the disconnect that slowly grew between him and such a romantic dream as he gained more power and lost trust in people.
My favorites would Crocodile, Doflamingo, and Blackbeard. Not sure who's #1. I wouldn't have any of the admirals up there just yet, since so far they're still only just interesting/threatening.