Ultron, for pretty much the same reasons that Digi said about Carnage, except for the thing about Ultron only seeing you as another number. Then there's also the possibility that no one will ever know what happened to you. Then again, I'm not too sure that I'd feel more fear from one or the other. You can only die once right.
It's tough to pick one villain, which is strange, now that im thinking about it.
But, for Marvel, i would have to say that the "X" titles have had more villains that seem to have been created with more focus on inspiring terror and horror.
In DC comics, i think that the arrival of Super Boy Prime would create a rash of soiled underwear.
Then, of course there r the independent books over the years.
Seeing the Plutonian = evacuated bowels
IMO, Dr. Doom would actually be one of the LEAST scary villains to meet. Just be sure not to insult him, and stay out of his way, and he won't hurt a civilian.
Guess it'd be one of the supernatural ones... Dracula, Bloodscream, Mauvais, or such. In fact, yeah, Mauvais. Getting eaten alive would suck.
Foom would look ridiculous irl. Hell, he already looks like that in comics most of the time.
How many black dragon penises has he sucked? Over 9000?
https://41.media.tumblr.com/ccd27a65f8c388d4f559e8cff9e8dd28/tumblr_nulfa27fQw1s2lswpo5_1280.jpg
Besides, it'd be a quick death.