I voted for
#3 . Here is my reasoning, which is just a matter of my personal opinion.
I just really love the combination of the blues, and the background seems to have a sort of space-like quality to it. To me this image looks like a snapshot of an intro scene before a battle. As they introduce each pokemon it shows them alone with an unreal type background, along with their name. (Imagine with me, the text flows in as they say his name, and he moves around and does something cool.) The kanji adds to this effect for me, I'm not sure what it says but it adds an authenticity for me. The light in the center (I guess that would be on his right wing) I see as something that moved over him during this cut-way intro, sort of like a spotlight.
#1 I like the colors, but it is very bright and busy feeling. That is probably what the artist was after in this one, but it feels a little too busy and muddled. I wish the pokemon was a little more distinguished from the background.
#2 How cute is Pikachu and Raiku? I love the background and the cute faces, but the one thing that kind of ruins it for me is the electricity in the center of them. I understand the purpose, and I like the electricity implemented into the background but as a focal point it is off for me.
#4 This one is my second choice. It makes me happy to look at it. The blue swirls and blurred pokeballs indicate movement to me. I feel as if pikachu just jumped down and knocked the balls out of his way. I like the use of the pokemon font as well.
#5 I'm with Wucas on this one. I looked at this a long time and I can't tell what it is. I like the dark colors, and appreciate the use of a less-mainstream pokemon, but I really can't tell where he ends and the background begins.
#6 The concept on this one is really cool, I like the silver look, but the I don't understand the text, and I don't think the font style goes with the image. Really if this was just a larger image of the pokeball with the reflection in it that might have been enough, I really like that idea. But as it is right now, the pokemon doesn't seem like he is in the right location to make that reflection.
Super entries, I hope there are many more to come in the future!