I don't believe in any sort of anthropomorphic "God" that actively manipulates human affairs and is responsible for creating the universe, etc. Now if you want to use "God" as a sort of label for some vague concept of "whatever we don't know, and will probably never know, about how the universe came to be, and why the laws of physics are what they are" etc., then sure, talk about "God" all you want. Personally it's not something I find to be particularly relevant to my day to day life.
Why do I believe this? Because incredible claims require incredible proof, and I haven't seen any proof for any of the claims about deities that various religions make, that don't have a better answer rooted in scientific knowledge. And while I don't hold science to be perfect, I find it more credible because science makes testable predictions, whose results have allowed us to manipulate the world we live in, in tangible ways. See: electricity, automobiles, jet airplanes, cruise ships, etc., etc.