If you want the highest Base Stat Total, then Mega Mewtwo (both X and Y) are tied at 780 each. Ignoring the Mega's, then a lot of legendaries hit 680: Lunala, Solgaleo, Hoopa (Unbound), Lugia, and Mewtwo.
If you want it for each of the given stats, then keep reading.
HP: Wobbuffet with base 190.
Attack: Mega Mewtwo X with base 190, or if you ignore Megas, then it is Deoxys-Attack with base 180.
Defence: Mega Slowbro with base 190, or if you ignore Megas, then it is Deoxys-Defence with base 160.
Special Attack: Mega Mewtwo Y with base 194, or if you ignore Megas, then it is Deoxys-Attack with base 180.
Special Defence: Deoxys-Defence with base 160.
Speed: Deoxys-Speed with base 180.
So in other words, the Deoxys forms win overall!