Has to use either colourless, water or electric energies.
Considering the letters "e" and "x" are capitalized, I'm going with the assumption that you are talking about the EX cards introduced Next Destinies and later.
Besides Mew grabbing a "Flip a coin until you get tails" attack, which can potentially do infinite damage, the EX that can deal the most damage is Mewtwo EX http://www.serebii.net/card/nextdestinies/098.shtml which can do up to 2360 damage with X Ball (this is never going to happen though, as it requires the opponent to have 59 energy in their deck and only one Pokemon and have all 59 energy on that one Pokemon. And then you have to have only Mewtwo EX and 59 Energy, and all those energy attached to Mewtwo.)