I'd say BG2 as you can import your BG character into the game. Plus, ToB is coming out so that'll add more stuff to an already an awesome game.
But I must admit that I've not played IWD/HoW, so maybe I'm biased.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc