Pokemon Legends: Keldeo.
We would return to Unova during the colonial era, to see the arrival of the malign original dragon in the region. The three Swords of Justice would try to defend Unova, and the dormant power of their pupil, Keldeo, would be the only salvation. The protagonist would have to help him develop his power, and defeat and divide the original dragon. In the process, however, the three Swords of Justice would break their bonds with humans and Keldeo.

All of them.

Seeing the other suggestions the Ho-oh and Zygarde ones seem the most interesting. Rayquaza seems the most popular but I feel it already got the focal point treatment in Omega Ruby and Alpha Sapphire where Mega Rayquaza was introduced.