Ultimate 747 gambit
The ultimate 747 gambit is an argument introduced by Richard Dawkins in Chapter 4 of his book The God Delusion. The argument questions if God is a good explanation given the assumptions established the 747 Junkyard argument. This is an attempt to subvert the 747 Junkyard argument and intelligent design, thereby making them self-refuting.
"However statistically improbable the entity you seek to explain by invoking a designer, the designer himself has got to be at least as improbable. God is the Ultimate Boeing 747."
- — Richard Dawkins 
Similar ideas about the weakness of design as an explanation were stated in David Hume's earlier book Dialogues Concerning Natural Religion . Hume also uses the idea of improbability implying non-occurrence in his essay Of Miracles.
- 1 Formal Argument
- 2 Counter Arguments
- 3 References
Dawkins does not explain how exactly the 747 Junkyard argument should be applied to God. He does state that his version "comes close to proving that God does not exist". The direct application of the argument to disprove God is as follows:
- If God exists, God is an ordered system and more complex than the universe he allegedly designed.
- From the 747 Junkyard argument, complex systems either originate from design or chance occurrence.
- If God exists, God was not designed.
- From (2) and (3), God originated by chance or does not exist.
- From the 747 Junkyard argument, the occurrence of complex systems by random chance is improbable.
- From (4) and (5), God probably does not exist.
However, given the summary at the end of the chapter, it is also likely the argument is intended to undermine God as an explanation:
- The order in the universe requires explanation.
- We will adopt the most likely explanation (or scepticism).
- The universe is a complex and ordered system, which has existed for a finite time.
- From (3), the chance of the order in the universe arising by a random process is negligibly small.
- From (1) and (3), an explanation other than random chance is required.
- If a designer exists, the designer (God) is a system that is ordered and more complex than the entity being designed (the Universe).
- From (4) and (6), God has less chance of random occurrence than the universe.
- From (2) and (7), God is a less satisfactory explanation for order in the universe, when compared to chance occurrence of the Universe.
In other words, using God as an explanation suffers from infinite regress. Dawkins goes on to say evolution is preferable to both chance and design explanations. Erik Wielenberg wrote an alternative formulation of the argument. 
A typical defence is to assert that God is a simple.  This depends on equivocation of "simple" being defined in a theological sense rather than the sense used in the argument. This prevents the improbably argument from applying to God. By defining God using obscure terminology, Dawkins commented apologists are "defining themselves into an epistemological Safe Zone where rational argument could not reach them because they had declared by fiat that it could not."  No simple system (in the everyday sense) has ever been observed to design a complex system. To arbitrarily insist on exceptions to generally accepted rules, such as the principle that a designer is more complex than their designs, is a form of special pleading.
A system cannot be less complex than any one of its parts. If God has complete knowledge of the entire universe, he must be at least as complex as the universe. David Hume pointed out that a simple God would be incapable of thought or emotion. 
God may also be defined as spiritual or having some property that makes the usual meaning of complexity irrelevant:
"But of course God is a spirit, not a material object at all, and hence has no parts. A fortiori (as philosophers like to say) God doesn't have parts arranged in ways unlikely to have arisen by chance. Therefore, given the definition of complexity Dawkins himself proposes, God is not complex."
These are are common beliefs about God, but without evidence this is another ad-hoc special pleading defence. The definitional defence can also be a form of Loki's Wager in that complexity is claimed to be undefined in the case of God.
God formed by a non-random process
The argument is based on God being formed by a random process. This is difficult to establish by evidence or argument. However, it is in keeping with intelligent design's tendency use the false dichotomy of order or design as the only causes of order. The hypothesis that God was designed would likely be rejected by most believers, leaving chance as the only possible cause.
God is sometimes considered as a necessarily existent being,  however this is simply defining God into existence without any basis. If the universe is assumed to not have this property, it is a form of special pleading.
It undermines the argument but does not disprove God
William Lane Craig said "At most, all that follows is that we should not infer God's existence on the basis of the appearance of design in the universe."  Craig only addresses the summary at the end of the chapter and ignores Dawkins's argument completely.
It is unreasonable to require an explanation for every explanation
William Lane Craig said of this argument: "If we required explanations of explanations, we'd end up in an infinite regress and we could never explain anything." However, the argument requires no such thing and his point is a straw man.
The universe still needs explaining
The argument is based on complex systems arising from either design or change occurrence. The argument concludes that a designer is improbable. Therefore, order in the universe arose from chance but is unlikely to be naturally occur because of its complexity; this is an absurd conclusion since the universe actually exists! It is likely that the cause of order in the universe (and arguably God) is something other than chance or design.
In reality, Dawkins does not accept the false dichotomy of design or chance. Dawkins based the argument on similar axioms as the 747 Junkyard argument in an attempt to subvert it. To disprove the Ultimate 747 gambit on these grounds also undermines the 747 Junkyard argument and intelligent design, which is probably what he intended.
Disproves a God that no one believes in
Most monotheists don't believe that God had to come into existent but was necessarily existent instead. This is rather an ad hoc assumption but since it cannot be discounted, it effectively sidesteps the ultimate 747 gambit. 
- Richard Dawkins, The God Delusion, p. 114
- Erik Wielenberg, "Dawkins’s Gambit, Hume’s Aroma, and God’s Simplicty," Philosophia Christi 11 (2009): 113-128. 
- Richard Dawkins, The God Delusion, p. 184
- David Hume, "Dialogues Concerning Natural Religion", Part 4 
- Alvin Plantinga, "Science or naturalism? The contradictions of Richard Dawkins", 12 Apr 2012 
- William Lane Craig, "The New Atheism and Five Arguments for God"