It's damn near universally considered a perfect rpg. BG2 is good but storywise isn't even close. I play rpgs. That is my preferred game type and I have never found it's equal in games.
I'll ignore the fact that Dragon Age was even mentioned as being better.
I play RPG's too and Planescape is easily under BG2, DA:O and others in my opinion.
As for universally considered a perfect RPG, if you type in "BG2 is the greatest rpg ever" you will find articles from Gameinformer and others stating it is. You'll also find the same results when you google "FF7 greatest RPG ever", or "FF6 greatest rpg ever", or "Planescape greatest RPG ever", or any other classic/currently highly rated RPG. Everyone thinks their favorite RPG is the best RPG ever, and nobody is right or wrong.
It's all personal preference and you can ignore whatever you want.
Also, GameStop not sending me the download code for the 20 dollar PSN card I purchased to get Okami, I purchased it like an hour ago. Normally it takes a few minutes or a bit but not this long. Hopefully I get the code soon so I can star the download so that I can play tomorrow.