I think I would choose the Needler from the Halo series. Basically, it does double the damage since the spikes explode after entering their victim. It also looks sexy as hell with the whole purple vibe going on.
I think I would either choose the Pumpkinhead Keyblade or Way to the Dawn (Riku's Keyblade). I just think keyblades are super cool, and I mainly chose those two because I like the way they look and Riku's one of my favorite KH characters.