My favorite rebuttals are some jokes I have put together throughout my life.
1. Religion, like a drug, is an escape from reality and like a drug can become addictive. Just say no to God.
2. In the beginning. Man created God in his own image, therefore we could say that God is the ultimate extension of the male penis.
3. God is the adult version of the childhood Santa Claus.
Slightly more offensive:
Religion is like a penis: You can wave it around and be proud of it, but don't shove it down my child's throat.
Religion is like alcohol: the more you indulge, the less coherent you sound.
Jennifer, That was an excellent one liner to catch those door knocking prosletizers off gaurd.
Andrea, When the Jehovah's Witness's come knocking at your door, you can always remind them that their founder predicted that the world would end in 1914. He was a nutcase much like the one who predicted the world would end back on May 21.
When the Mormon's come knocking on your door, you can remind them that there is no evidence supporting anything that Joseph Smith said. There is no evidence that Jesus and Isrealites ever visited North America. The DNA evidence, from native Americans proves that Joseph Smith was lying.