Here are four core arguments against God's existence that have persisted since ancient times. Using these four arguments as an organizing principle, I present for your consideration the Four Horsemen of 1872-1939.
Lack of empirical evidence- Discovery trumps revelation. This is the skeptical or naturalistic argument. As Sigmund Freud observed, religion is inconsistent with objective observation.
The argument from inconsistent revelations- Sectarian religions are mutually contradictory and internally inconsistent. George Bernard Shaw observed that the religious will cherry-pick indefinitely rather than abandon the notion of divine revelation altogether.
The argument from evil- If the gods are real, they have a lot of explaining to do. Emma Goldman rejected the notion of a capricious god with the inclination to force subjects into submission.
The argument from reasonable nonbelief- If the existence of the gods were self-evident, everyone would believe. The burden of proof is on religion. As Bertrand Russell famously observed, there are any number of absurd propositions that we do not take on faith.