Well, I've only played three games and in each one it's been different. I kind of like that, actually, since it sort of keeps things fresh. I haven't played my Yellow or Crystal versions in a while and really cannot remember what my signature Pokemon would've been for either of those (although I suppose I could just pop one in my GBA and find out since I still have those files). However, in my Diamond version it was definitely my Luxray. Crobat comes pretty close, and actually I remember it being one of my best in Crystal, too, so maybe that would be it. :>