Scary LOOKING race/monster, I mean... not scary because we know their stats and know they can destroy us. Because, let's face it, beholders are bad *ss, but if I saw one floating towards me, I'd probably end up laughing myself into a ball on the floor while it killed me.

As far as what race/monster I think is scariest, I'd say dragons or trolls. Dragons is a no brainer... but trolls, what could be creepier than a creature that looks like that and having it walk towards you, healing it's wounds even as you beat it? Creepy stuff there man...

A close third is dwarven women... those beards... *shiver*

Mindflayers

Especially after they stun you and all you can do is watch a tentacle-face suck your brain out

The Skeleton Warriors look a bit scary when you first see em!, so much bigger than those ones in BG1...all that changes when you start fighting them, they just fell to bits! Waaah no fun!

Drow I'd have to say. A typical conversation would go like this;
Me; What's an elf like doing in a cave like this. Ow stop hitting me! Let go. No not the whips no no!! AAAAAAAAAAAAAAAAAAGH!

Humans have always been a bit scarier than the competition.

I felt a shiver running through my spine when I watched a picture of a Rakshasha. Possible Spoiler;
~
~
~
~
I saw that head of Ihtafeer (Trademeet Dao Djinni's) and those eyes being sown together... Brrr ~shudders~

Mindflayers...I HATE MINDFLYERS..I HATE MINDFLYERS I ARRGGGHHHHHHHHH!!!!!
*Ahem*..Exscuse Me...
Xen

the scariest things are bone golems. Dragons are more impressive than scary (well ok they're scary as hell, but only after you stop going "oooooh ahhhh")
adamantine golems are pretty scarey looking too.

on the other end of the scale, Im sure kuo-toa are a tougher opponent than I think, but they just look so stupid I cant respect them, and smash them without mercy. same with the silly shark-heads.

ps does anyone else think githyanki (I know I cant spell it) look like the protoss from Starcraft ?

I think Minsc is scary when he informs you he hid Boo from his captor...
As for scary foes, I think I'd be afraid of just about any creature if I suddenly woke up in the FR world. Except perhaps for regular animals, halflings, elves, half-elves and not too nasty looking humans. Maybe I could live with gnomes as well, but most of the rest would scare me. Even kobolds.
And once your pants are wet, it doesn't matter who did it anymore.

.
.
.
(late game spoilers...)
.
.
.
beware!

The only creatures that really freaked me out in the game were scary not because of how they looked but what they did. Before chapter 4, those critters were pit fiends and liches. Before chapter 6, those were illithids. Thing that freaked me out the most: adamantine golems. Those things pack a punch, let me tell you.

The atmosphere really disturbed me in places. Suldenessellar creeped me out. Beautiful elven city with the sounds of inhabitants defending their homes. Really touching and pathos-invoking at the same time. And don't play Hell at night. The sounds. Creepiest thing in the known universe. Heck, even Minsc turned yellow in the Fear test.