One of the strongest and most popular arguments against the existence of God is the 'argument from evil.' According to this argument, the traditional god people believe in can't exist in a world with so much evil -- either God would do something, or God can't be very good. This is, in fact, one of the oldest arguments against the existence of any gods, not just the god of contemporary western monotheism. Do you agree that it's the strongest atheological argument or is there something better?
For certain types of gods, some form of the Argument from Evil is probably the strongest we can have because it creates such a powerful contrast between the way the world actually is and the way the world should be if the claims of theists were even remotely true. This forces theists to engage in all sorts of redefinitions in an attempt to salvage their beliefs, but the more they redefine the weaker their position actually becomes.
Even worse, they often move further and further away from their traditional, orthodox religious beliefs, thus undermining any attempts to argue that their religion is true. This is a problem seen in a lot of efforts to prove the existence of a god: at most, apologists argue for the existence of some sort of vaguely defined god that is removed from the universe, but this god comes nowhere close to the very involved god of traditional Christianity.
Even so, as I noted above this is one of the oldest arguments against gods and has been used against even non-omniscient, non-omnipotent, non-omnibenevolent gods. Although this argument cannot definitively prove the nonexistence of such gods, it does suggest that their existence is unlikely and, if they do exist, that they are hardly worthy of worship. This is important because once we pull the "worthy of worship" rug out from under a god concept, much of what's normally taken for granted is lost and the entire structure of a religion built around obeying and worshipping gods starts to crumble.