Seneca comes to mind immediately. In the 47th letter in his Epistulae, he argues that slaves should be treated not just humanely, but as equals. It's a beautifully written text and it's rather short, so it's definitely worth reading in its entirety, but here is an excerpt:

"He is a slave." His soul, however, may be that of a freeman. "He is a slave." But shall that stand in his way? Show me a man who is not a slave; one is a slave to lust, another to greed, another to ambition, and all men are slaves to fear.

Aristophanes is not a philosipher but a playright, he wrote this in one of his plays.

Praxagora: I want all to have a share of everything and all property to be in common; there will no longer be either rich or poor; [...] I shall begin by making land, money, everything that is private property, common to all. [...]

Blepyrus: But who will till the soil?

Praxagora: The slaves

A joke, but shows that this was a topic a lot of people thought about to some extent. even if the didn't come to the same conclusion that we would.

Gregory of Nyssa is the ancient figure most openly opposed to slavery that I've ever encountered. He writes in opposition to it in his homilies on Ecclesiastes, arguing that it's not simply wrong because of cruelty, but by it's very nature:

‘I got me slave-girls and slaves.’ For what price, tell me? What did you find in existence worth as much as this human nature? What price did you put on rationality? How many obols did you reckon the equivalent of the likeness of God? How many staters did you get for selling that being shaped by God?

If man is in the likeness of God, and rules the whole earth, and has been granted authority over everything on earth from God, who is his buyer, tell me? Who is his seller? To God alone belongs this power; or rather, not even to God himself. For his gracious gifts, it says, are irrevocable (Romans 11:29). God would not therefore reduce the human race to slavery, since he himself, when we had been enslaved to sin, spontaneously recalled us to freedom. But if God does not enslave what is free, who is he that sets his own power above God’s?

I think Gregory is just about the only person to argue this way in antiquity. He's a remarkable thinker in many ways really.

edit: in case it's not clear, the "I got me slave-girls and slaves" is the Biblical passage that Gregory is commenting on.

Can't think of any clear cases of people speaking against it off the top of my head, but as always its a nuanced thing.

Aristotle, Politics: "For that some should rule and others be ruled is a thing not only necessary, but expedient; from the hour of their birth, some are marked out for subjection, others for rule..."

Aristotle is pretty adamant that some people are just meant to be slaves. He concedes that occasionally a free man falls into slavery due to random misfortune, and thinks that man should be set free. Considering that manumission was pretty common with Greek slaves, it would be quite easy to think, hey, if you've got the spirit of a free man, you will work your tail off to buy your freedom, you have the ability if you deserve it! (Not unlike the American notion that the poor should pull-your-bootstraps-up to rise out of poverty, if you are a hard worker and smart.)

Plato in my opinion, has more of a benevolent and paternalistic attitude about slavery, but still feels that its natural. In the utopia he presents in the Republic there's a strict caste system where the enlightened philosopher-kings make the decisions for the state. So the other castes aren't really free, its like a benevolent oligarchy... but the rulers are kind and just and best-qualified, so it's like they are slaves with really good lives. This kind of echoes what he says about slavery, mostly that Greeks should only enslave non-Greeks and not be cruel to them, to help the slaves learn to be better, etc. Remember that several centuries before Plato Greeks DID enslave other Greeks, usually because they fell into debt. c.600 BC Solon freed Athenians who were enslaved due to debt. So Plato would have been aware of this and it probably softened his stance on slavery.

Ancient Greek slavery was generally a more favorable condition to be in than Roman slavery, which itself was sometimes more favorable than slavery in America, the Caribbean, and Latin America, which is the most familiar concept of slavery for most of us. The opportunities for freedom for a slave in Greece (and especially Athens around Plato's time) were a lot better, the work was often crafting instead of heavy agricultural work or mining. If Athens had a system of slavery like a Brazilian sugar plantation, its possible that their philosophers would have taken a stronger stance against the practice.

This might be a better question for /r/askphilosophy (Not saying it's inappropriate here, just that philosophers may be better at providing context for their reasoning and argumentation)