From a design standpoint, I really like Necrozma (the base form). I mean upone first glance what do you see, some sort vaguely humanoid crystal monster. But the more you look at it the more you realize how mangled it looks, the legs are backwards with upside down feet while also looking atrophied, the elbows and shoulders are locked in an unnatural and uncomfortable looking position. The oversized head is vaguely reminiscent of a cranial defect. The face only superficially looks like a face where it's nothing more than a pattern on a prism that glows.
And the sharp edges and shapes give an unnatural and alien feel to Necrozma. A trait that for some could be considered a negative, but for Necrozma it works in its favour of emphasizing just how wrong the existence this creature is like an experiment gone horribly wrong or a being from an entirely different plane of existence. Then there's the whole thing where if you were disassemble and reassemble Necrozma's body you'd get a Dragon's Head, yet Necrozma looks nothing like a Dragon helps further compound Necrozma's unnatural and horrifying air.
Additionally Necromza has the paradox of being entirely black, yet reflecting light, despite the fact that blackness signifies the absorption of light, not reflection.
All this I feel comes together beautifully to make something that comes across as a bizarre beast that may not even qualify as being alive.