IMO
In terms of brokenness via gimmicks, it goes:
Dynamax > Megas > Z moves
Dynamax was just inherently a broken mechanic. Doubling your health for 3 turns alongside of effectively 3 turns of Z-moves, all of which either buffed your own stats or caused special effects like Rain or Sand was just too much.
Megas were very cool in concept, but the issue is that Pokemon that didn't need the help got them anyway, so you have blatantly overpowered **** like Metagross or Salamence in the same sentence as Audino, Abomasnow or Absol.
And Z-Moves, while being delete buttons, are hands down the weakest of the gimmicks. Yeah, they're delete buttons, but they're one use, and you sacrifice your hold item. If you don't delete the opponent in that one hit, your Pokemon is just inherently worse off now.