Faith Isn't Quite Brain Surgery
Guy: How can you believe in God? You've never seen him.
Girl: Well, I believe you have a brain, though I've never seen it!
–F train via Overheard in New York, Feb 28, 2006

Oh snap! Sorry, thought it was funny so I had to post it. Seriously though, that's one of the worst arguments I've heard for not believing in God.