My ideal VN would be much more of an SoL with real life elements to them (and more preferrably, with romance). I like reading stories with minimal fiction elements. Charactes should be solid. The protagonist, the heroines, and even the side characters, should leave an impression to the reader, rather than just being there. The setting and concept of the story should be good. Good art is also a plus, as well as interface. And most of all, a well written story that has an impact. I might actually play VNs not having anything to be proud about except for its good and impactful storyline. For me, it is the life and soul of a VN itself.
Clannad is right on the mark, though I still think that some other heroines leave much more impression than Nagisa, and I still think Sunohara is the best side character ever.
My ideal VN might be closest to G Senjou no Maou. Good characters + good art + epic story + impact + Maou is really awesome!