Killington has legit expert terrain especially woods like Julio, and trails like Ovation and some stuff in the Canyon.
Add Smuggler's Notch to the list. Add Sugarloaf to the list. Add Magic Mountain to the list.
Stowe (plus Mansfield)
There are other areas that have some difficult terrain but mostly depending on conditions I think those are the ones that stand out.
IDK how EpicSki is saying these East areas don't belong on the list. I've skied a bit out west and IMO tight steep eastern trees with variable conditions make for some of the most challenging stuff to ski.