Imo the Flayer is scarier cause Vecna is just a born psycho who loves torturing while the MF is a mysterious giant spider-shaped smoke monster/entity that tower over Hawkins.

I will have to go with Vecna

Vecna is definitely more terrifying than the Mind Flayer. He's totally pure nightmare fuel!

Venca is pretty weak as the big bad of the show. Should have been something like the Mind Flayer or something native to the Upside Down.