Most Broken ZA Mega?

but in mega do do acual damage its huge

Mega Zygarde would have a tough time against Mega Clefable… just saying the obvious.

As for broken Mega's… that's up to the beholder. If you knew all their abilities and listed them here, then you'd probably be able to notice ones that are going to be truly scary. Mega Dragonite having Multiscale pretty much permits its base form to run Inner Focus. That allows it to safely Mega-Evolve because Inner Focus prevents Intimidate and Fake-Out, giving it a means to get rolling on Turn 1 unless it runs into a Mold Breaker user(Mega Emboar for example) that can bypass its ability and deal Fake Out damage while neutralizing its Multiscale ability(making it easier to KO Mega Dragonite).

Mega Meganium has a pretty good ability too with Solar Beam firing in 1-turn without the need for Harsh Sunlight and Mega Feraligatr can turn its Thrash move into a stronger Outrage via its Dragonize ability(aka Pixelate for Dragons). Not sure how much use Mega Froslass will see given that Abomasnow basically does the same thing but without the need to Mega Evolve and I assume Mega Eelektross will retain its Levitate ability(making it a pretty strong option unless you run into Mold Breaker).