I vote Jirachi. It's adorable, powerful, and best of all, it grants wishes! I always have wishes that I know will probably never be granted, and I always dream about Jirachi granting them all.
RUNNER UPS:
2. Dialga and Victini
3. Rayquaza
4. Keldeo Resolute Form
5. Zekrom/Black Kyurem
6...