It's impossible to say what the worst argument is, for there is a continuum of arguments, progressively worse, and at some point, we have to say that what we have is a rhetorical flourish that can't fairly be handled as an argument. ("You only think God exists because you were raised to believe that!")

But I guess you could ask: What is the worst argument put forward by a credible philosopher? In that case, Dawkins probably doesn't make the cut.

I find it very hard to say, actually. I suppose I think Russell's argument that causation does not apply  outside of the universe quite bad.

Russel's argument is pretty bad, but I've looked more at Dawkins's and his makes me curl my hand into a fist. He says there is no satisfactory explanation for God, and he says this without considering the works of Aquinas, Leibniz, Plotinus, and co who use metaphysical principles such as PSR to say that the explanation for God is satisfactory in His own nature. What arrogance he demonstrates. What really ticks me off is his argument for why God must be "complex" in the fact that He is omniscient, and must be able to hear all prayers, which Dawkins state must make Him more complex than a brain/computer, which seriously just shows us how much of a straw man he constructed(assumes God is a material being), and why his argument is worthless. Even looking at dirt is better than looking at his garbage. But unfortunately he paved way for other gnus such as The Atheist Experience to start using this argument as if it is a huge blow to theism, when it does not even make a dent in it. He arrogantly assumes his argument without any metaphysical premises, and does not even bother tackle the arguments for Divine Simplicty. In fact I'm pretty sure he doesn't even know what it is. I think for me what makes it so bad is how arrogant he is about it(and the fact that on wikipedia it is listed under "philosophical arguments, which is a disgrace to philosophy). Sorry if I keep ranting about Dawkins, but it is because of him one my good friends turned into an intolerant gnu who hates religion for superficial reasons(while thinking that they are rational reasons put forth by one of the greatest "philosopher") who keeps praising him, and thinking that he is some sort of "savior" to her.(Plus ruining my chances of dating her as this anti-religous stuff she pulls is a huge turn-off)
 

Well I wanted to see if Dawkins is as bad as people say he is, and now I can confirm for  sure that he is worse than that