I'm not actually a big fan of the SETTING of the last stage (not great, but tollerateable for me) but it's definitely the best in terms of gameplay.
I had a good bit of difficulty with AI and pathfinding. Right around when starting on the FER stage I really solved some issues and started to create good pathfinding. That is why FER has the most complex paths and platforms out of all the stages, which in turn makes it an interesting stage to play.