To me PS:T is much more than just an interactive book. It has an atmosphere and setting that for me has not yet been matched in any other game. It had a party where each party member had a background that actually influenced the game in one way or the other.
As all games it has it's downsides also, but for me it still ranks at the number 1 position.
However from what is known now about Riftrunner I fail to see how it's to be compared with Planescape:Torment.
