Hmm.. There is a recurring issue with the thumbnails, and that's size recognition. Without other elements she's not reading as small or short. I'd recommend throwing in something else in there that suggests her size. ^^
From a compositional view, I think 1 and 5 looks the most interesting. All of them work though, but you'd have to move the subjects around a bit more to make them really pop. Not take up too much of the canvas, and leave enough negative space. 
1. with the right background elements this one could be really cool, maybe move the witch a tiny bit to the right though.
2. You don't want her head that close to the top of the canvas. The comp works, but only if you add perhaps about 1/3-1/4 extra space on top of the canvas.
3. It's a bit boring, but to make it work, move the character to the right, so she's not standing right on the border of the canvas, with her staff in the dead center. Also, maybe add a tiny bit to the top.
4. Move her a tiny bit to the right, perhaps move the cat down a tiny bit, and you're set.
5. If I read her pose right, this one could be really cool, but you need to either move her to the right, or widen the canvas with 1/4.
6. This one is probably the worst one. She's over 2/3 of the canvas, she's going to appear as HUGE. You need way more negative space to make something like this to read. Also, she's way too close to the top of the canvas. ^^
Hope this helps you narrow it down, or help you push the one you prefer to something awesome 
