Evidence first, for this is what came first for me. It is also the simplest to express. Quite simply, everything I have ever experienced makes more sense to me in the absence (versus presence) of a deity. Furthermore, everything I know of the world - including religion itself - makes more sense to me in the absence of a deity. Personally, I don't rate "personal revelation" as a particularly reliable or trustworthy source of evidence. (As a scientist, quite the contrary in fact. We wouldn't need statistics otherwise.) When it comes to "evidence" for the divine/supernatural, I have simply never encountered any other kind - and even experiential "evidence" has been limited to that of other people. (And not from lack of trying, I might add.)
The lack of evidence is probably reason enough for me to be an atheist but I think it is important to acknowledge that there are other factors too. I think people often assume that atheism is a very negative and reactionary thing. Whilst I admit that my rejection of Christianity was negative, I see atheism itself as very positive. It is, I believe, embracing the true nature of the universe. There is more than enough beauty and wonder in this reality for many lifetimes of exploration and curiosity. There is power in shaping our own destiny, purpose and meaning. I just have no need to look to ancient texts for these things, although is interesting to see what ancient cultures thought. I feel like I have been there, tried that and life's too short. I prefer to spend my time pondering mysteries that might be solved in my lifetime. If I'm lucky, I might even solve some myself! This is really what I mean when I say that I don't need a deity/religion (although I also mean that I don't feel the need for one to "explain" anything nor to define morality).
It goes beyond just not needing a deity, though. I have no desire for one. And this lack of desire itself goes beyond my personal philosophical position that, if a god exists, it is either mean or insiginifacnt, and I am interested in neither. For me, a deity or supernatural element belittles and diminishes the true wonders of the natural world: the human brain; the way that unseeing, unthinking and uncaring molecules can interact to produce something as amazing as an ant, or a cat, or a person (or a bacterium etc. etc.); the way that unseeing, unthinking and uncaring cells and molecules can ultimately evolve into something as amazing as an ant, or a cat, or a person (or a bacterium etc. etc.); the truly awesome size of the Universe; the enormity of time; the insignificance of Man in the grand scheme of things but the ultimate significance of a man to a select few; true altruism. You get the idea. I know that my religious friends feel sorry for me and think that I am missing out on something but that's OK: I feel the same way about them.
In the same way, it goes beyond just not needing some externally imposed sense of purpose or meaning or, yes, morality. It is actively desirable that these things come from us, from people, explicitly and with all the limitations that comes with. No (sane) person ever claims to be omniscient. We will not get man-made ethics and morality "right" - there is no absolute "right" - but we can do our best and change our best in the light of new findings or situations. In the (hopefully immortal) words of Tim Minchin: "I don't go in for ancient wisdom. I don't believe just because ideas are tenacious it means that they're worthy." I'm proud to apply 21st Century knowledge and reasoning to 21st Century problems and come up with morality and ethics that may be flawed but can be explained and reasoned and changed if necessary.