It's a definitive role-playing experience, and the only reason it can't be called the best game in its class is because in a sense there's nothing available that compares to it.
The story itself, the vast unique maps, the missions, the NPCs, everything really. You can do almost anything, and you can do it your way.
Here is a good review of the game.
http://www.gamespot.com/pc/rpg/baldu...baldurs%20gate