Full Idea
My own suspicion about the Ontological Argument is that the fault lies in taking notions like 'the most perfect, impressive and powerful being conceivable' to be well-defined.
Gist of Idea
I think the fault of the Ontological Argument is taking the original idea to be well-defined
Source
Colin McGinn (Logical Properties [2000], Ch.2)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.50
A Reaction
I'm tempted to put it more strongly: the single greatest challenge for the theist with intellectual integrity is to give a clear and coherent definition of God. There must be no internal contradictions, and it must be within the bounds of possibility.