Make your own mega and rate the megas of others. Mostly whatever you like but try to be reasonable:
>Try to not use non-existant abilities
>Don't minmax unless its NU or below
>Don't increase HP

I'll start with mienshao:

HP- 65
ATK- 165 (+40)
DEF- 60
SPA- 135 (+20)
SPD- 60
SPE- 145 (+40)

Ability: regenerator

I dumped some points into Special attack to make it a bit more realistic, wasting some points like GF usually does, what do you think?