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Megas weren't necessary for the Battle system or anything, but it was a fun concept that made older Pokemon exciting to play with again.
Z-Moves were less appealing as an idea, since they're mostly just generic super attacks. But they did have a few interesting applications here and there, like Z-Conversion, Z-Stockpile, and other status moves with crazy secondary effects. There was room to get creative if you wanted to.
Dynamax is trying to be this weird middle of the road feature, but it loses any appeal that the other two may have had. The tranformation is basic. Instead of making an individual Pokemon better with improved stats, types, abilities, etc., it just gives everyone fat HP stats and 3 turns of overpowered Max-Moves. And Max Moves, unlike Z-Moves, no longer have that fun variety of effects based on the move being powered up. Normal Status moves will always be Max Guard, Fire moves will always be Max Flare, etc.
Megas >>> Z-Moves >>>>>>>>> Dynamax
