Pikachu was my signature Pokemon when I was much younger. I used to use him, unevolved, on ALL of my Pokemon game teams. I was pika obsessed.
I have no idea who my signature Pokemon would be now. I use all different ones on my games now, and haven't used the same pokemon on more than one team in years. I haven't really used Pikachu in a long time, too, and when I do, I tend to evolve it into Raichu now (like on my y file).
Having Lucario as a new signature could be cool, though, if I find opportunities to use him on more games. He is Korrina's signature Pokemon, and I like/admire/crush on Korrina a lot as of now. :)