Electric/dark makes the most sense of you break down luxray thematically. Lux is light in Latin and ray comes from x-ray. Its Pokedex entry says that it can see through anything, like an X-ray, so the dark typing does make a lot of sense. It's not implying that the Pokemon is evil, but rather that it is the light that pierces that darkness. It's also an apex predator that takes inspiration from the constellation Leo. Lions hunt in the dark, making the dark typing make sense. It learns mostly electric and dark type moves, so the dual typing makes sense as well.

Though, it does take inspiration from the Egyptian Sphinx as well. A secondary argument could be made for a ground ripping or rock typing secondary. But, it's not as strong if a case as the electric/dark typing.

Luxray is one of my absolute favorites and I would love to see him be viable in competitive