I'd say, Daggerfall and Fallout2. Daggerfall has such a brillaint features as Spell-, Item- and PotionMakers, Guilds with their ranks, and, of course, skills system - I think it is wise that the character gain better skill levels through frequent using of the skill. On the other hand, Fallout has one of the best combat systems. Plus, these games have no those nasty classes (though Daggerfall actually has classes, you can create your very own class simply choosing appropriate skills, and, if you please, advantages and diadvantages, thus creating whatever PC you want).
And BG2 has magnificent story, the best NPC's and...well, you know about all the pluses of BG-SoA
... it is just irresistable in involving into the gameworld.
------------------
Tanda,
with best regards from Guild of Assassins