While i do not have any particular strong argument against god, i do have a pretty overused one:
WHY, why is, and was, so much war, hate, and so on, if there really is a supreme benevolent deity that watches over all the Universe, and particularly the earth, why doesn't it do ANYTHING? Aren't most of the wars because, while most people agree on the existance of a supreme being, they don't agree on what exactly might his/hers/its name is? Shouldn't God then just poof over into everybody's house and say "my name is *such*, you can now stop every war, and so on, about me. stop hating everybody, too."