If you're using that background, then yes, the red and gray one in the middle-bottom is probably the best, though I'd try out the others just to see.

 http://i.imgur.com/hkZEjfX.png what do you think of that?

The top-left one is actually pretty good. Also I like the colors on that one because it looks quite poisonous. Mwahahahaha!

My only concern is that these are being looked at out of context... what does the background of your game look like when you're fighting this boss? That will have a huge effect on how good they look in-game.

Bottom middle looks best to me.

I like the bottom row, also top right, but they all need dark/black eyes.

second image of the third row is the one man, the other ones are either too eyestraining or don't pop out enough.