Most feared enemy?

[Captain Pancake]

For me, it's the regenerators from resi 4. God, those things were creepy. it's THE BREATHING. the constant DEEP BREATHS. also, the fact that you couldn't kill them without a rifle and the thermal scope, unless you were a lucky shot and had a TON of tmp ammo...

 

[Arcadia2000]

Everyone's already hit all the major ones for me. Since I tend to stay away from games designed to creep me out because I don't like to be creeped out, that leaves the darknuts from Zelda, and swimmy things that are faster than you, regenerators from RE4 and the bag-head chainsaw freaks (Hey, I don't play it but I watch others play it and they still give me the jibblies.), and pretty much anything invincible that you just have to run like hell from. I don't like those.

Blue shell. D:

 

[Worgen]

nemesis, you think youve got resident evil figured out then suddenly something is following you tho doors

 

[HaussVonHorne]

Terrorists in Call of Duty 4: Modern Warfare on Veteran Mode.

 

[Grounogeos]

Dark (or normal) Grenchlers from Metroid Prime 2.

<img src=http://magatroid.com/griever/wp-content/uploads/2008/10/grenchler-300x236.jpg>

They're about as awful as their name suggests, they shoot lightning from their horn, move with near-impossible speed for their morphology...and of course they bite. Somehow they're even more dangerous than the Space Pirate commandos, which makes little sense if you think about it....

I'm not even going to get into some of the sub-bosses in that game *shudder*

Grenchlers aren't scary, they're cool. Almost as cool as the Sheegoth from MP1 (ignore the bad pun; it was unintentional).

I'd have to say ReDeads (except the ReDead Knights from Twilight Princess; those ones I have absolutely no problem with). The ones from OoT looked creepy (their artwork was straight-up nightmare-causing), made that dreadful moaning noise, and the scream made me (the PLAYER, not Link) freeze.
The ones from Wind Waker weren't much better. At least they don't moan. They're not as scary-looking... when they're dormant; that gaping maw and those red eyes freak me out. And their scream is even worse than the OoT one. Plus, when you use a ranged item on them, they turn and look at you; not a pleasant situation.

And let's not forget that the ReDeads from OoT/Majora's Mask/Wind Waker attack by trying to rape the back of Link's head.

I also really don't like the parts of Matroid Fusion where you have to run from the SA-X, especially that one part in Sector 2. I was too afraid to jump down and run for it, cuz that thing can rip Samus apart (the fact that everything goes silent except for the SA-X's footsteps and that weird whirring noise doesn't help).

 

[Blind0bserver]

I had to play Guild Wars a lot recently while researching an article, so the bane of my existence for the last month or so has been The Destroyers.

Destroyers are giant fiery-cored monsters that come from a pit of fiery unpleasentness somewhere deep in the center of the earth (not Hell, apparently a different fiery pit of unpleasantness). They're large, they're powerful, they're ornery and they're always at least eight levels above the level cap for players.

Sure, you can acquire a skill or two that makes fighting them a little easier, but fighting them is a pain in the ass no matter how you look at it. I mean look at that thing! It is at least ten feet tall, and that's when it's slouching.

 

[Instant K4rma]

The damn boss in the Shadow Temple in Zelda: OoT. The first time i saw that thing, I was 8, and it gave me nightmares. Even now that im 17, that thing still scares the shit out of me.