Purely based on aesthetics, Cofagrigus. That may sound like an odd choice, but a big, possessed coffin flying at you? Uh, no thanks (honorable mention to Gigantamax Sandaconda, but that's because I'm terrified of snakes IRL)
Factor in Pokédex entries, though, and as we know, there are quite a few terrifying ones, but Gengar is by far the worst IMO. Pretty much all of its entries are pretty scary, but worst of all is it's Sun entry: "Should you feel yourself attacked by a sudden chill, it is evidence of an approaching Gengar. There is no escaping it. Give up."
That's such a gloomy outlook. Like, the Pokédex literally tells you just to accept death if you're possessed by a Gengar. Even worse, if it's a ghost type, you probably won't even see it coming.
Seriously, Gengar is completely messed up.