I might normally say deathclaws, but those are really the ones that are the most dangerous, not the ones I'm afraid of. My most feared Fallout 3 enemy is definitely the feral ghoul. I'll admit, I'm quite the coward, and if I walk into a dungeon and hear that "RAAAGGHH!" sound of a ghoul, I get the hell out of there. I know I can probably mow down a whole gaggle of ghouls with a few strokes of my shishkebab, but there's just something about them that terrifies me on a primal level. Like I said, I'm a coward, and it took me a long time to get used to going into dungeons where enemies could jump out at me. I can just barely tolerate the suspense. However, this doesn't happen at all with human enemies (raiders, mercenaries, enclave, etc.). I guess I'm just used to seeing humans all the time and they don't register as a that big of a threat in my subconscious mind.