I played 4 matches the other night as Greninja against M Mewtwo Garb, testing specifically for that matchup and the new promo. The only game of the 4 I won, my opponent failed to get out the Garb until too late. I have retooled Greninja and am going to try some more.
Pretty much every deck that doesn't rely on abilities is running Garb. You can pair it with Yveltal, Darkrai, Scizor, Mega Ray, Audino, Gardevoir (except that uses Dragonite now), probably even some others could as well. Basically any pokemon without an ability runs Garb because there are so many Volcanion and Greninja out there and carrying Garb against those is almost an auto win.
But what I really want to know is why the algorithm pairs Garb more frequently with Greninja and Volcanion than it does other decks. That's the real question here.