Clarifying or restating the case for atheism

If you don't allow a definition of "God," then the conversation is a waste of time, just as any philosophy is a waste of time if one of the participants refuses a definition.
My view is that there might be "superman" types out there in space, who can work wonders and who would seem gods to us -- not unlike the Greek pantheon, but they are not "God."
I also think there might be a force or presence in the universe that makes sentience and so on possible, but forces and presences are not God either.  This is just avoiding admitting that you don't really believe in God either.  Some honesty in the discussion is necessary.
The deity in question does not have to be the Abrahamic God -- heaven help us if he is as this God seems rather evil -- he could be Brahma or Zeus as many of the ancient philosophers perceived him -- the key is the word "omnipotent."  The thing is if he is not omnipotent he is not God, but superman, if he is God he is irrational because omnipotence, as even the Scholastics understood, leads to self-referential logical contradictions -- otherwise known as reductio ad absurdum proofs that the premise is false.
Now the "standard" resolution offered for this is the assertion that God is omnipotent -- he can do anything -- except things which he cannot do.  The problem with this is that this limitation applies to every motivated being.  Indeed, I am omnipotent if you allow such an exception -- I can do anything except what I cannot do.
This is what is behind the assertion that God is irrational.  There is also the clear evidence we live on an earth where life is dominated by the principle of natural selection, and natural selection implies constant fear and suffering and disease and death.  To me this is the "problem of suffering" and rules out any deity with good intentions.  A de fiat creation would have been kinder than God using this method to create life.
It is fairly obvious to me that these arguments are not understood by some of the participants here, since the responses are not to the point.  All I can say is that if you don't understand the arguments about God, you have no business having opinions on the subject.  
Regarding Buddhism, the Buddha was (if he existed historically) pretty much an atheist in the sense that he did not think there was a god, but he didn't care much -- he said our problems are the same as those of the gods if they exist -- the problems of existence and karma and suffering -- so gods are not relevant.  As Buddhism evolved it accepted existing rituals and deities of the areas it entered, as part of its generalized notion of tolerance that goes beyond just getting along but includes allowing "worship" of extraneous deities in temple by those so inclined and the withholding of criticism or question by the monks, who are trained better, of what those untrained may do.  I think this historical practice has been wise, but it does have its problems.