These are great:

www.iamanatheist.com
Some of the best “arguments against”:

Hitler was an atheist.
“I don’t know about Hitler’s religion, but I do know that he was heterosexual, so can I assume you’re against that, too?”

Einstein believed in God. Do you think you’re smarter than Einstein?
“If he believed in God, yes.”

There are so many wonderful things in the world, how can you say there is no God?
“It’s really pretty easy: ‘There is no god.’ See?”

God made His image appear in this tortilla!
(Eats tortilla)

www.iamatheist.com
Some of the best “arguments against”:

How can God be omniscient, omnipotent, all good, and the creator of all things when those qualities lead to logical contradictions?
“It’s a mystery.”

How can Jesus have risen from the dead in three days if the first day was Friday? Wouldn’t the third day be Monday?
“It’s a mystery.”

How can God, Jesus, and the Holy Spirit possibly be three separate entities and still be one God?
“It’s a mystery.”

The Bible says that men are the masters of women.
“That’s true.”

The Bible says you should not eat lobster.
“That part’s only for Jewish people.”

Why are there so many contradictions in the Gospels?
“There are no contradictions in the Gospels.”

But the Gospels can’t even agree on how many people showed up at Jesus’ tomb.
“There are no contradictions in the Gospels!”

Your arguments aren’t very convincing.
“Enjoy your eternity in Hell!”