It's very difficult to make one pithy statement that ends the discussion on whether any gods exits. It's far easier to assert the non-existence of one specific god, yahweh for example, by pointing out logical inconsistencies with that deity's characterstics.
What I generally say is that when I look at the millenia of recorded history and thousands of gods and religions that humans have been involved in, I find it ludicrous to point to one spot on the scroll, whether it's the 5th century BCE in the middle east, or the 3rd century CE in Europe or the 7th or 16th century wherever, and say "THAT'S the real god we need to follow". it's not particularly strong, but I try to get them thinking.
I have a lot of Evangelical friends, and the subject of my aethiesm comes up alot. In fact I encourage them to challenge my beliefs, because when you get past all the nitty gritties I can always return to one core truth. If you can not question your beliefs, and come up with answers to those beliefs, then you have no business having a belief in the first place.
No theist can claim absolutely never to have had doubt, both in himself, his faith and whatever flavour of divine being they have chosen. If you stick to this principle, and have worked to Aethiesm from a Theistic standpoint. Then no matter the argument you can always win against a Theist, or at the very least cause them to pause for thirty seconds as you hurriedly leg it :-)
Bingo! That is always my argument. How can a god that loves us soo much, watch and do nothing while children are being abused. If he does exist, then why doesn't he do something to stop the abuse? I never get an answer for that. Then I get, who are you to try to understand god. People have to find the answers themselves.