Why is the ontological argument of God's existence the best argument?
An ontological argument begins with a definition of God then argues from the basis of that definition to the logic of the existence of God, as Anselm's and Gödel's ontological arguments do. These arguments employ no empirical evidence and proceed on logical reasoning alone. Refutations of ontological arguments--a class of argument beginning with a definition, proceeding on logical reasoning and ending with the conclusion of "necessary existence"--is possible such as in the challenges of Aquinas, Hume and Kant who rejected the ontological based on human inability to know the nature of God (Aquinas), the absence of evidence-based reasoning (Hume), and rejection of the state of existence as a predicate (a defining term within the ontological definition) (Kant).
With this in mind, one possible reason the ontological argument might be called the "best" argument for the existence of God is that the ontological argument does not proceed from evidential substantiation and God, being super-spatial and -temporal, embodies no empirical evidence for evidential argument. There are those, however, who approach proof of the existence of God from scientific arguments. Dembski posits an information science argument that leads to intelligent causation of original information. Dr. Amit Goswami posits a quantum physics science argument that leads to a cosmic consciousness that authors unlimited possibilities that actuate specified information through quantum channels.
For those who think the ontological argument is the best argument to prove God's existence, and not all do by any means (Kant and Hume are two who did not think this; Dembski and Goswami currently do not think this), two reasons for it being best have to do with the non-empirical nature of God.
God's nature is not empirical. There is no physical evidence about God that can be gathered and analyzed. Argument must be a priori, dealing with reason alone and not with evidence.
Ontological argument is logical reasoning that starts with a definition and ends the reasoning process with a conclusion stating a necessity of a proof, in this case the necessity of proof of God. Since God is spirit and not physical, it may be deemed that this means argument fits best.