Yeah, I'd say Baldur's Gate has the best overall experience, but the story in Planescape: Torment blows everything out of the water. Seriously some of the best writing I've seen in any game to date, plus it actually has themes that the game explores and fantastic characters. Dak'kon alone has one of my favorite quotes ever:
"When a mind does not know itself, it is flawed. When a mind is flawed, the man is flawed. When a man is flawed, that which he touches is flawed. It is said that what a flawed man sees, his hands make broken."
EDIT: Oh, and Neverwinter is amazing for the sheer amount of user-made mods. And these aren't just "mods" like other games where they just add items or tweaks to gameplay, these are the old D&D definition of mod, meaning module. They're full-fledged adventures and campaigns with their own stories. Some of them are absolutely amazing - check out the NWN Vault and get the top-rated ones. You will literally get hundreds of hours of gameplay well beyond the contents of the actual game made by BioWare, and a lot of it actually surpasses the work that BioWare did.
And really, the Hordes of the Underdark campaign was pretty good, too - well worth the price of admission alone.