On the hijack--I'll be interested in VeloB's opinion, too. I am also a Spinervals junkie and have Hillacious, Uphill Grind and the Tucson Training Rides.
Personally, Mt. Lemmon is the toughest. Unrelentless. Mean. Tough.
Then, Hillacious and Uphill Grind. I can't say one is harder than the other; they're different. Hillacious seems to feel like it is working me better because it is more like I ride--long upills, rollers and short steep sections. But, Uphill Grind really gets the heart rate up, but in a different manner. It feels more like learning to sprint up a hill than just to get to the top. Both are tough, but differently so. I alternate them, tossing in Mt. Lemmon when I'm having a good day.
Gates Pass is the easiest. I don't consider that a hill workout so much as a tougher aero with some hills thrown in to raise the heart rate for a while. But that could just be the way I ride that one--aero the flats, work the hills.