I very much enjoy the original IWD, for many of the reasons listed above by others.
The music is some of the best in any game, especially the Kuldahar music.
The art in the game is beautiful. It really makes the environments within the game pleasing to look at, and really draws you into the world. Also, the character portraits are my personal favourite of any RPG. I usually import IWD portraits into other RPGs (like NWN, who's portraits I didn't like).
I liked creating the entire party at level 1, which was a nice change in pace from other RPGs. While I certainly enjoy other types of games (as said by others, BG2 and PS:T are also my personal favourites), IWD is an extremely clean combat RPG. I really liked being able to choose exactly what kind of party I wanted.
One thing that I probably like the most about IWD is the level design. The different dungeons are all memorable and really cool places to visit. Dragon's Eye was varied enough, but Dorn's Deep and The Severed Hand are my personal favourites in the game.
The cutscenes in the game, which more or less separate the different chapters, are wonderfully done and really draw you into the atmosphere of the game.
Another thing: dialogue options. While your different options don't have much to do with the overall progress a lot of the time, there are certain NPCs that you can be incredibly sarcastic too, which is awfully fun.
I'm currently playing IWD2, and enjoying it. I think I like the first more, but I'm not really sure why yet. It really depends on what rules you like, be it AD&D or 3e.