Playing IWD1 after the BG series was interesting. Although it's not my favourite game (I played it twice), but I loved its music and when playing it first time, the creatures that were not in BG were interesting.

I liked the Frost Giants, nice work of the graphical designers! The long beard, and giant axe, and the clothes... looked great altogether.
The ghosts were good as well.

What about you?

I think the Hook Horrors (were they IWD1?) were quite good. Sure, they dont look too good (just a black shape, iirc), but it was something a little different from "humanoid. with sword".

I think that the best monster is the frozen beholder in HoW

The Cyclops are fine work either!

Favourite bad guy? hmm toughie. I know I hated the trolls.. heh. But the monsters I most enjoyed engaging where the shadowed orcs and goblins. Was really creepy fighting shades, and they were a lot harder than the ones you meet at the start.