Lavos' appeal is his Lovecraftian nature. He's a giant literal space flea who is going to end the world because it eats planets for breakfast, and his mere presence on the planet has changed the very fate of all life on the planet for better or for worse, and he's not even doing anything except what he naturally does. It's not really a surprise that Jenova took so much inspiration from him as well while combining it with a Lovecraftian aesthetic.

I'll come back to this one. I have one character I would love to talk about but he doesn't neatly fit into the "villain category" hell I can't even really say he's an "antagonist" either.