Giygas, from the Mother series, namely EarthBound. While I personally find him to be more awesome than disturbing he's proven to a lot of people that he is quite horrible. He is the embodiment of evil and has no real physical mass when he attacks, Ness and his party cannot even comprehend his attacks, it is as if they are ethereal, yet do tons of damage.
Also, here's some infamous 'music' to go along with his encounter.