If you didn't already have Planescape: Torment, I'd say get that, no question. Since you already own Planescape: Torment and Baldur's Gate 1, I would suggest either Baldur's Gate 2 or Neverwinter Nights.
BG2 is very similar to BG1, but more refined and much more expansive. The world is gigantic and intricate (you can spend well over 100 hours exploring the game's nooks and crannies even before you dig into most of the real meat of the storyline), the interface and character design is significantly refined, the story is great, and the character advancement is more robust. Baldur's Gate 2 is traditional 2nd edition DnD at it's absolute finest.
NWN struck me as very odd coming from the Baldur's Gate series, because you only had your one main hero, and a single henchman NPC that you could take with you rather than a full party of 6 adventurers. If that doesn't bug you, check it out. It uses 3rd edition D&D rules, and it's specifically designed to support player-made mods of all shapes and sizes. The original game was very long, if somewhat lackluster at times; the expansions were somewhat shorter, but better. The player-made campaigns can be absolutely incredible.
Of the two, I personally preferred Baldur's Gate 2 because of the party interaction and diversity instead of being so focused on your main hero. The Icewind Dale series is also quality dungeon-crawling, but it's much more combat-focused and generally much harder. Not so hard as to make you ragequit in frustration after dying a dozen times in a row, but it's a significant challenge. NWN is the easiest of the three.
Post edited September 23, 2011 by DoctorKumquat