Charizard y, i like the changes, its got giant ass wings, its sleeker and longer, it looks a lot more badass, and it looks more like a glass cannon which is what charizard is known for being, looks a little more draconic too.
x is blue and black, it has red eyes, it has flames coming out of its mouth, which is nice but.....none of it makes any sense, don`t get me wrong it still looks badass.
I mean y is less of a drastic change but its change that makes some sense, i like it better overall.
In terms of battle, honestly both are good i use each of them in equal measures.