I like III. The idea that there was a Light and Dark world and that Darkness does not equal evil was a nice first for the series. Also, the fact that too much Light caused such dramas 1000 years ago was a shock for all those Light = good people. I like the idea of balance between the two worlds, and the effects of a disturbed balance were quite noticable in III: the surface world was frozen in time for 1000 years (making Xande the most succesful villain in the entire series). The entire balance thing was a nice twist and a kick in the behind of everybody who said "darkness must equal evil!".

IX had some very interesting and relatively deep ideas as well. But that game was just a masterpiece, so that is to be expected.