1. Beholders (I refuse to use the shield of Balduran)
2. Vampires (they always drain my cleric, causibg him to forget his restoration spells...)
And, only in BG1, the Ettercap. I know we're talking BG2 here, but they deserve a special mention IMO. Boy were they tough back then
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The last arrow of Legolas kindled in the air as it flew,
and plunged burning into the heart of a great wolf-chieftain.
All the others fled. -J.R.R. Tolkien, The Lord of the Rings