1. No Christians after the Rapture! This is assuming that they are right, which is completely unlikely. But even so, could an atheist imagine a better world than one in which all the 'true believers' were suddenly whisked away to free the world of religious division and hatred and allow mankind to put our effort into improving the real world and this life for a change!
This page also offers some pretty convincing arguments. A couple of my favourites:
...you'll save money on funerals because you now know there's no point in getting all dressed up, since there's no place to go!
...you'll get to finally think for yourself, according to a recent scientific journal.
In truth no reasons are necessary to be an atheist (though they do exist!). Atheism is a default position, we are all born as atheists, starting out in life without any belief in deities. The burden of proof lays with the theists, it is they who must persuade us of the existence of their god and the validity of their religion.