Madman
Out of all the signatures here, yours is the most interesting. Does it win on the effects used? No, does it win on quality? No. But your IS the most interesting in when you look at the composition. Batman has always been a dark figure, the anti-hero. The flames and the added effects brings this out to a great extent in this signature. The flamey background (and the colors) mixes in exceptionally well with Batman. And the 'blood spatters' (intended? Or blending of the flames layer?) make it really gruesome.
The downsides of this signature are the more technical aspects though. Obviously, I am missing a black border, essential for good boxing. On the left side there is a weird rectangular which messes with the flow of the rest of the signature. Weird white cape part on the right side of the signature. Would look better if Batman wasn't opaque.
Viper
Not sure what you exactly tried to do here. Red is not the first color I would associate with Batman, though that may just be a difference of opinion. However, the render had some really nice and dark color shading going on, by making the entire signature somewhat mono-colored, you took that extra 'oomph' out of the render and made it more static. It is also very hard to distinguish Batman from the background. Though that might have been your entire point, I would have gone about a different way of doing that. Most likely messing around with black levels rather than a monotonous color.
The grungy background does work with Batman, and the black parts of the signature's background almost make it look like there are hundreds of bats flying over there. Can't help but think that it would have looked better if there was more color in the signature though. It could've look like Batman was in the Bat-cave.
Typography is not very interesting. The font is rather bland and the black color is hard to distinguish from the background. I had looked at the signature at least 3 times before I even noticed there was any text there at all.
Snowman
I'm going to be straight to the point here; you ruined the render. The American drawing style has some really neat shading lines going on, but you sort of...blurred the entire render. Not sure what you were trying to do with it.
Also, I'm not sure if I am seeing it correct, but were you trying to add some grain, film-noir style to the render? I can actually see that working really nicely if done properly. What I also really like is how you added that black spot in the top left corner. Emphasizes on that dark atmosphere of Batman. Sadly, the white part on the lower right corner sort of contrasts too much with it, and doesn't work with Batman.
The bats are a nice touch, but they too are of low quality, and look rather blurred. Though when comparing the blur factor with the render, they look less blurry than the render, drawing your eyes more on the bats rather than the render, giving a sort of reverse Depth of Field effect.
Typography isn't interesting here either, scanlines don't really work for texts, color is somewhat okay, would look better if it was slightly more yellowish (though I could also be wrong, because of the scanlines). I would personally also go for a different font.
Demonoid
You have some interesting effects going on here. As far as I can see it, you took part of the render and motion blurred it? What I like about it, is how the motion blurred background nearly looks like rain. And it nearly gives me the feeling like Batman is standing on top of a building in the rain, gazing down on the citizens of Gotham City. It doesn't look sharp enough to be actual rain though. If you could sharpen the lines a bit, it would look a lot better.
The glow effect you added on Batman also make it look more like a constant flow of rain is falling upon him and the rain disintegrates into a sort of mist. I would have gone with a different color of the rain though. However, you can also go about a different way with this; if you darken the background a bit, and Batman even more (by playing around with the (black) levels), and extend that glow a bit more to the left. You can have a little spot-searchlight going on here. And as we all know, Batman has his very own Bat-spotlight, so it would again add to the atmosphere.
I somewhat like the typography here, even though Comic Sans is one of the most hated fonts around (seriously, when you can; don't use it), I do like the blending of the font. Simple, but it remains readable.
That is how you give a comment on someone else's work.