Tin Woodsman said:MRG for stuff within the boundaries or on the trail map. For stuff you need to hike to or find on your own, it's Stowe hands down. The Kitchen Wall, Hellbrook, Tusk and certain other shots are as difficult as anything you'll find anywhere, and they require just a lift-assisted hike. There's really no comparison in the East until you start talking about true BC skiing like Tucks.
Tusk doesn't require a hike perse, well, I guess you do have to hike down to it depending on the snowback. Because the snow is almost always sketchy there at best, I find that to be the most difficult shot on the mountain - more so than Hellbrook or Hour Glass.
In bounds, no hike required at Stowe - Pipeline gets my vote for most difficult. 1000 vertical feet of six feet wide 45 degree pitch nastiness. If you're one of the first 3 people down it on a powder day consider yourself blessed. It can only handle a handful of people before all the pow is scraped off at which point, its near impossible to make it down without hugging a few trees. That chute is far and away the most relentless experience I've come across within a ski area's boundaries on the east coast.