Ash probably has a good shot at getting Mew, Mewtwo, Celebi (several), Suicune, Latias, Latios, or Darkrai, since he befriended them all in the past. Latias is the most likely (you all know why), but also he has saved the life of several Celebi (Celebi movie, random pokeranger episode in sinnoh, zoroark movie), so he could have a good chance of getting those to help him too.
White FC: 1850 1893 8931
I use legal, obtainable hacks
I might upload the battle to YouTube (username N1jaSk1lls
). Tell me if you don't want me to.