Arcanum, easily. The strength of Baldur's Gate 2's writing lies in the companion conversations. Outside that, the actual roleplaying choices and consequences are decidedly lacking compared to the likes of Black Isle and Troika games.
Arcanum plays like a spiritual successor to Black Isle's Fallout games, and so most quests can be solved in multiple ways. Add in the fantastic character creation system and its industrial-era steampunk world populated by orcs, humans, gnomes, etc., and it's easy to see why Arcanum is regarded to be highly ambitious.
IWD series features the best combat in the Infinity Engine games, and the atmosphere and music are memorable. The actual plot, though, is merely passable.