in my opinion the best argument against God is the argument that he’s just one of many possibilities, and therefore unlikely to be true.
when you think about it, God is a gimmick answer. people wonder what created the universe and so forth and so naturally people think “It was made by something that’s capable of doing it.” it’s not really a real explanation. it’s not even a satisfying one.
and, of course, if you’re not going to ask where God came from, you have to wonder why you’re invoking God to begin with. why not save a step and just assume reality has always existed then?
there are all sorts of real-world arguments against him. our world falls in line with what you would expect if there were no God.
at some point though people invent gods because it’s the intellectual response to the physical fear of death. we’ve also got a built-in desire to seek safety, for being taken care of, and a God makes us feel this.
anyways though for most religious people their beliefs aren’t based in argument or reason, so it usually does no good. it doesn’t matter how much evidence you have, you can always say your god is above it. that, of course, results in an unfalsifiable god.
the next step is to point out why an unfalsifiable belief is a poor idea. the flying spaghetti monster is unfalsifiable too and yet you don’t believe in that, so why god? if your belief is one possibility of many, and there’s no way to prove him right one way or the other, do you really think it’s that likely to be true?
i think most people understand this but don’t care. they LIKE believing in their gods and quickly reject any reason that chips away at it. it’s a desired self-delusion.