I think one of the best arguments against the existence of God for me, is that God's omnipotence and his omniscience are mutually and internally self-contradictory. According to Christian doctrine, God's omnipotence means he has the ability to do anything whatsoever, or at least that is what omnipotence formerly meant (theologians have since watered down its meaning). Also according to Christian doctrine, God's omniscience means that he has perfect and infallible knowledge of everything, including past, present, and future. This means that the future that God has infallibly foreseen is so certain that it is as if it has already happened. It is set in stone.
This leads me to ask the question, does God have the power to change the future, or anything else, for that matter, from the way he has already infallibly foreseen seen it ? If he can change what he has infallibly foreseen for the future, then he didn't infallibly foresee it, and therefore is not omniscient after all. And if he cannot change what he has already infallibly foreseen, then he cannot be omnipotent after all. This goes against the law of logic called, the law of non-contradiction.
This, I think is one of the strongest arguments against the existence of the traditional monotheistic, Abrahamic God. As for a deistic type God, he / she / or it, is said to have set the universe in motion, and since then has taken no more action, and does not intervene in the universe, leaving no traces of His / Her/ or It's existence. Such a God, then, might just as well not exist, if it does, indeed, exist,