I'd argue z-moves are the most balanced mechanic as they're really just a "kill what's in front of you" button. Hell, z-moves were way more balanced than something like the gems from gen 5 because they got rid of a move's secondary effect (most of the time) and you can only use them once. They didn't overcentralize the meta, they were just kind of there. You could even build a successful team without using z-moves at all, which is more than I can say for the (more or less) mandatory inclusion of a mega on your team during that time. Compare that to the matchup button or Mr. "doubles your stats". No one complained about z-moves because they were more or less just banned. Tera was nearly suspect tested and dynamax was outright banned because they were so ridiculous and powerful and stupid that people didn't want to play in a metagame with them.mega Pokémon are the most healthy and balanced gimmick.
edit: I also ranked how healthy all of them are
Megas
Dynamax
Tera
Zmoves
whatever miridon does
The correct ranking here is z-moves, mega, dynamax, and tera.