BG, PST, and IWD are the hooks, of course I do like features from many others (FO games including NV, NWN2, etc).
BG was just such a good fantasy RPG that would pave what I believe made a good RPG.
PST was so full of depth, and showed me that a good story could make a game over everything else.
IWD while more linear than the others had some of the most fun straight up dungeoneering.