My vote has to go to 6, it's the one game in the series where I feel everything really came together, outstanding intro/graphics (for the time, although the intro is laughable now!) great story, fun puzzles, good voice acting (Goooood day Prince Alexander!), different paths to reach the end which were actually very different, and even a song to go with it!
As good and even ground breaking as the earlier ones were, it's 6 that really stands out for me.
Also as inm8 has said, Jane Jensen + Roberta Williams, can't argue with that!
