If you're REALLY into RPGs you should get ALL of them:
Baldur's Gate Compilation (4 in 1: BG1, Tales of the Sword Coast Expansion, BG2 and Thron of Baal Expansion)
Icewind Dale: The Ultimate Collection (3 in 1: IWD1, Heart of Winter Expansion, IWD2)
Planescape: Torment
Neverwinter Nights Diamond Edition (3 in 1: NWN, Shadows of Undrentide Expansion, Hordes of the Underdark Expansion; possibly even including two Premium modules: Kingmaker & Witches Wake)
Neverwinter Nights 2 Gold (2 in 1: NWN2, Mask of the Betrayer Expansion) AND Storm of Zehir Expansion
OR if you live in Europe Neverwinter Nights 2 Deluxe Edition (3 in 1: NWN2, MotB, SoZ)
and perhaps the Mysteries of Westgate expansion (online purchase only though)
As for the best story, as others have already pointed out, that's most probably Planescape: Torment but be aware that it's very different from a console game and requires lots of reading. BG2 has also a great story. And NWN2 seems nice, too, although I haven't played through it yet. Keep in mind that NWN and NWN2 are more than the official single player content, if you own them (and preferably teh expansions, too) you can also choose from a large collection of community created stories, a lot of them even better than the official content, all free for download. And you can play on online worlds with old or new friends without having to pay a fee.
All of the games mentioned above are based on the D&D rulesets but on different versions of them. If you're looking for other and mostly older (Action-)RPGs on the PC, you could also check out some of these:
Diablo 2
Throne of Darkness
Titan Quest
Nox
Revenant
Arcanum
Rage of Mages 1&2 (part strategy games but which CRPGs aren't...)
I guess you don't want to go back to 80s and 90s classics like Ultima, Dungeon Master, SSI Gold Box Games, Eye of the Beholder, Realms of Arkania etc.?
Well, they're probably not sold anymore anyway...
Okay, maybe you should just check out NWN & NWN2 first.