Owlbears- Because despite any silliness that may or may not be inherent in the concept, they can be pretty terrifying in-game. The power of a bear, and the mad, relentless pursuit of some crazed bird of prey. The owl features make for a nice dose of creepy.
Gnolls- Hyenas are already a pretty frightening creature in the right setting. They cackle unnervingly, and their jaws are among the strongest out there.
You'll see the gleam of their eyes as they huddle around some unidentifiable carcass. Mix all that pack instinct and inherent spookiness with bipedal movement and a culture that has them draping themselves in the body parts and items of their fallen prey, and you have a striking adversary.
Kobolds- Traps. Cunning. Their almost comical frailty and cowardice leads the party to assume they'll be easy to manage, which makes it all the more shocking to them when the kobolds actually do gain the upper hand. They're a good way to lure the party way over their heads. Especially with a dragon tossed in the mix.
Wraiths- They could come at you from anywhere, and the damage they traditionally deal is far more grave than sword slashes.
Hybsils- Not sure I can explain why these are so appealing.
Barghest- They think they're fighting goblins. They think they're fighting wolves. They're not.
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)
So much fun...