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Don't have a specific set of favourite megas but Lucario (I know) and CharY are up there along with Sceptile, Ttar and others. I like most of them though to be honest.
As far as where i'd put the mega stone though? That i'm sure of... I would put in a watch like in Ben10 so every time I wanted my mon to go mega i'd shriek "IT'S HERO TIME!"