they did a good job with base necrozma. i feel like i could go on for a long time about why it's my #1 fave.
making a nice creature design out of a prism is hard enough, but they really hit it out of the park in my opinion. i love the idea that the face is just a very fortuitous arrangement of shapes. yet even though it has an unmoving face and very limited movement in general, the animators manage to convey its feelings and personality perfectly through body language. and of course, i just think the design is cool.
i also liked the ultra beasts and silvally...i was a big fan of gen 7 in general. it got me back into the series and may have dethroned gen 5 as my favorite.
Ultra Necrozma with its music felt like a Final Fantasy boss. As with the ultra beasts, I initially hated the idea of them at first since they looked inauthentic, but since I finally played gen 7, I appreciate them a bit more now. Also, I'm a sucker for time and space travel concepts.