For me, it's the level where you fight the Harpies after retrieving Phineas' eye. I found that to be damned fun, infact possibly what this game was shooting for with it's aerial combat. Angel pulling from Harpy to harpy and Baltos to baltos, demon dodging, all while never touching the ground, fighting a group of opponents who are not helpless in the air (unless you clip their wings.)
Crackin' good indeed.
Crackin' good indeed.