Excluding legendaries and mythicals:
Bug is a tie between Mega Pinsir, Mega Scizor, and Mega Heracross, each with 600.
Dark is Mega Tyranitar, with 700.
Dragon is a tie between Mega Salamence and Mega Garchomp, both with 700.
Electric is Mega Ampharos, with 610.
Fairy is Mega Gardevoir, with 618.
Fighting is Mega Blaziken, with 630.
Fire is a tie between both of Charizard’s Megas, who both have 634.
Flying is Mega Salamence, with 700.
Ghost is a tie between Mega Gengar and Dragapult, who each have 600.
Grass is Mega Sceptile, with 630.
Ground is Mega Garchomp, at 700.
Ice is Mega Abomasnow, with 594.
Normal is Slacking, with 670.
Poison is Mega Venusaur, at 625.
Psychic is Mega Metagross, with 700.
Rock is Mega Tyranitar, with 700.
Steel is Mega Metagross, with 700.
Water is a tie between Mega Gyarados and Ash Greninja, who both have 640.