If there is a God (and there is), how it must laugh to watch mortals deep in heavy-duty thought debating its existence or it's form. I say "laugh" because any God that exists must surely have a sense of humor when it comes to humans.
Anselm: God exists because we can conceive of such a thing in our minds. The very conception of something means it exists. Thoughts are things. Just because you can't see something doesn't mean that thing doesn't exist.
Aquinas: So you really think you can conceive of God and what God is? Only God knows what the nature of God is. God exists, but not because you can conceive such an entity exists. Only God can think those thoughts with true knowledge of his nature. Your vague conceptions suck.
Descartes: I think, therefore I am. You don't have to know the nature of God. You only have to conceive that God is perfection, whatever his "nature" is. Give me a break. God exists because I can conceive of a perfect entity. Je pense donc je suis. Cogito ergo sum. Or did I say that already?
Relax Max: Dubito, ergo cogito, ergo sum. Contemplatum hurtus cabeza. I doubt, therefore I think, therefore I am. Thinking makes my head hurt.