I think the most challenging hoax is the one where nobody can even guess how it was done. Convincing CGI requires a great degree of skill as has been demonstrated in this thread. However, it's the stuff made by rank amateurs that can be the most challenging. And you don't have to use CGI to achieve convincing imagery that will suck in a large number of people. I'm thinking of the well know ATS hoaxer 'gridkeeper' and the videos created through a telescope by John Lenard Walson. Even now, even after being tagged a hoaxer, Walson's images continue to appear in new threads right here on ATS. There's at least one near the top as I write. I such cases, the explanation may be so obscure or so specialised it's either rejected without consideration or not even read. In Walson's case, so few people understand the ramifications of or have access to astrophotography, that the hoax is simply swallowed as real. You could argue the same technique is employed by David Sereda with his mile wide UFOs in the infamous STS-75 tether incident (which also rumbles on as I write). The only way to debunk such hoxes is by the application of the scientific method using established physics and by demonstration.

WG3

I think it was the first Jurassic Park film where I really saw the line between reality and CGI blurred for the first time. Since then the whole movie industry has changed (unless the Narnia films were made by hundreds of out of work, SAG card carrying centaurs).

That said, I believe the biggest problem with the UFO hoaxers is that they are limited by their imaginations (which tend to run wild and carefree). Without joining a 'side' here, let's look at what they do [the hoaxers]:

Implant into real footage the image of an 'alien' craft.

Now, if I am an alien and I've been studying the Earth for a while, and I've just flown across the galaxy to get here, am I going to sit out in broad daylight for all to see while I do my observing? If I were on an Army reconnaissance mission would I stand on a hilltop with a big orange vest on? If I were hunting, would I wear my white shirt and yellow pants in the forest? The answer is no, I'd deploy some sort of camouflage. The hoaxers don't take this into consideration. If alien craft are here, chances are you'd never see them. If they got here, they are pretty clever. Much cleverer than a computer nerd with a graphics card.

The other side of the coin is the debunkers who scream "it's a plane" every time they see a video of a reported UFO. "I can see the strobes/landing lights!". Well, if I were an alien and I needed to travel in plain sight, would it be a real stretch for me to turn on some lights to make my 'craft' look like a plane/helicopter? Just another form of camouflage, no? Again, they got all this way because they are clever. Having no form of defense - like camouflage - wouldn't be so smart.

If you're thinking in your own little box, you can't expect the world to be in the same box. Credit where credit is due.

As for the 'artists' who create the hoax videos, I'd be the first to congratulate them on creating great work if they were up front about it rather than claiming it to be real. We all saw the Blair Witch Project, and we're pretty used to that "ooooooh, it's real!!!!!![HA! Not really....]" marketing strategy.