VII or X.

AlthougH X suffers from much less mood whiplash than VI or VII. I need to finish VI one of these days but all I remember from it was how bizarrely it could go from goofy to serious. The Ghost Train thing was absolutely absurd but then it ends with Cyan seeing his family and you're like well that kinda sucks.

VII was the same way. One minute you're an ecoterrorist and then the next you're crossdressing.

X was the most consistently grim and dreary.