It is only non believers who ask for proof of God
I don't see skeptics asking for proof of God or even evidence of God. They're telling you that they would need that before believing. Sometimes that's worded, "Where's your evidence?," but that should be understood to mean that he knows none will be forthcoming.
The following are arguments against God:
1. The world has too much suffering for a benevolent Creator to exist
2. God by definition can't be virtuous since he can't be tested by courage or acquire value rather he is eternal and can't chose his essence.
3. God would be obvious to everyone and would perform miracles in the open if he existed.
These are three philosophical arguments I'm aware of. I don't believe them myself but they are formidable.
None of those are good arguments against gods. The first one is only an argument against a benevolent deity. Indifferent and malevolent deities are logically possible even with all of the suffering we see. The second assumes that a god must be virtuous, eternal, and unchanging to exist. Many religions teach that, but that doesn't make it a logical requirement. Why can't the creator of a universe be mortal? Maybe the deist god died in the act of universe-birth. And the last assumes that gods must be apparent to exist. It's not hard to imagine a deity that doesn't want to be found, or is unaware of our existence, or is uninterested in man, and simply never manifests.
Skeptics don't need arguments against gods to be unbelievers. They require evidence FOR them before believing.