First pokemon I got max IV's with and a careful Nature. I'd say RU, it really could use more physical defense. It suffers from 4 moveslot syndrome with bulk up/Milk Drink since it's grass type and can't really make use of it's STAB if it opts for Edge Quake or something. Due to Mega's being introduced, it's a lot harder to utilize a bulk up set on it since there's still plenty of pokemon that threaten it after one bulk up. It's still suceptible to Toxic too and has to worry about Dragon's/Flying types a lot (might be able to stop some fire types with Earthquake after one bulk up, but it'll probably be fairly injured).
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It's got a great movepool, but being a defensive Grass type isn't the easiest thing in the world. It also has some stiff competition with Shaymin and Virizion. I can imagine Gogoat working well in a team revolving around using Grassy Terrain. I'll be nice and say it will be UU for a while, but it might drop to RU.
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