Deny: "to state that (something declared or believed to be true) is not true". Dude didn't have a leg to stand on.
I don't know about the worst attempted proof I've ever heard (probably something about impossibly pretty sunsets), but here's my favourite bad one, courtesy of a well-known YouTube evangelist.
Here's a paraphrasing: If I say something will happen after an infinite amount of time, will it ever happen? No. Could there have been an infinite amount of time before the present? No, because if there were then we would never have reached the present. Therefore there has only been a finite amount of time, and God started it off.
The central problem is that infinity can end at a given instant. An infinite amount of time can have passed before the present if time extends backwards to negative infinity, rather than starting at a definite point. The same way positive infinity can start at a given number, e.g. (4,5,6,...), negative infinity can end at a given number, e.g. (...-1,0,1,2,3,4). Imagine two arrows starting in the same spot and pointing in opposite directions.
There are other issues of course. Even if the universe were finite, there are plenty of theories for its origin which do not involve gods (and often point to very eccentric properties of early time). Even if it proved a god, it provides no link to the Christian god advocated in the video.
However it's the way the argument makes a pig's breakfast of high school mathematics which really stands out to me as an occasional maths tutor. You have to wonder whether the guy genuinely doesn't get it, or he just hopes the video will reach others who don't, in which case the video is pure sophistry. Considering that he doesn't allow any critical comments, let alone official video responses, I tend to think the latter.