Well if I had to make a selfish decision I'd go with Tsuna from Reborn.
But in order to get new members and gain popularity, I'd say go with Grimmjow or Ichigo. I can't really pick one. I dislike Bleach, but I would gladly have a Bleach banner because it will help out g2 much more than a selfish choice of mine.
I guess I'll go with Grimmjow since it would fit with the current skin.