Mew: It's the oldest Pokemon ever, so it'd know a few tricks of the trade, if you know what I mean. XD
Celebi: It has the power to transcend time, so that means it could just go back in time until it won. ^.~
Jirachi: It sleeps for a thousand years only to wake up fror one week each thousand. Come ON, it couldn't even fight back. *gets killed by Jirachi lovers* >>
So, I'm going to say Mew or Celebi. Not 'Chi, 'cause 'Chi's not good. =P
Edit: Doesn't this need to be moved?...To Pokemon VP?