1. One is good. It has flow. Although, it seems a bit low quality. The border effects you have going just don't work. I'd either make them solid, or remove them. Also, the kind of monochrome thing you have going doesn't work nicely. either add in some more splashes of colour or completely make it b/w, although the former, me thinks, is a better idea. And whatever the orange things by lightning are don't work. Overall, very good attempt.
2. What it looks like, is you made a background, slapped on the guys, and then slapped on text. Easy peasy, doesn't look like much real effort. It looks more like it could be an advertisement for a FF site, rather than an actual signature (in that context, it's great). As is, it's okay.
3. Looks okay. Basically blurred city background, and placed Sora fanart on there. You could've done a little more to spice this up, such as star texture, or perhaps a C4D. I'd play around with this a bit more and see what you can do. Also, the colouring isn't the greatest. Try messing with it a bit more? (It appears to be a soft light/hard light effect, but I could be wrong)
4. Sora and the KH logo thing don't go together here. At all. The Sora thing looks really random and doesn't exactly fit with the text either. Also, the text. Don't use outglow on text. Just don't. (Unless you know exactly what you're doing, but for 99.9% of siggies, avoid it at all costs) It also needs to be placed better. I have a feeling you could've made a sig out of that text, and made the text the focal point, but that's just me.
5. Huh. A bit too bright. Also, I notice how the clipping mask/strip behind sora doesn't align with the wireframes and C4D's you have going. Not too bad, it just bothers me. I do like your colouring though. Also, that red effect to the right of Sora (just that wave-y thing part) should go.
6. I don't have much to say about this. A bit more colouring job would've been nice, if you were going for something simple like this. Also, see if you can make another copy of the renders and fill them with a single colour and play around with it. (I don't have any examples off the top of my head. sorry. I'd have to go look. ;_

like, move it a few pixels behind the renders, see what that does.
7. I have nothing to say about this. /zips mouth