The Baldur's Gate II engine is superior - fast walking, pausing on the inventory screen, better graphics, and the Tab key. (
) However, the original Baldur's Gate has a better main plot line, which I find sucks me in more right until the very last sword stroke. Shadows of Amn and Throne of Bhaal couldn't - the munchkin in me loves Angurvadal +5 and always will (he even made a mini-mod that I use to make it slightly better), but the plausibility lawyer in me wonders how the hell Tethyr still exists with greater earth elementals, mature green dragons, and Kuo-Toa elites running around everywhere.
Baldur's Gate II's side quests are also of highly varying quality. I like Nalia's keep (even though it's just hack and slash), the Planar Sphere, the Unseeing Eye, and Trademeet; but others, like the Shadow Thieves, are deathly boring. Also, when the NPCs talk too much, it rubs my imagination and my conceptions the wrong way and fills everything with "canon". (I'm aware that I'm very vain when it comes to my imagination and very reluctant to give up any of my pre-conceived notions but I'm not sure what I can do about it.)