There is no BEST expert resort. If there were it would probably be Silverton only because they offer only Black Diamond terrain.
Skiing today at Big Sky/MLB I was struck by the endless amount of different high expert terrain choices available. We skied 4 walk on tram laps (it looked like the vis was poor, it wasn't) Dropped the N. Face of Lone mtn into MLB, 4000 real vert. feet, the upper 2000' was pretty damn good. Then went round to the Headwaters lift and caught Jack Creek, probably one of the steepest, longest lift served runs in N. America. Some hiking required. Never even touched Challenger other than to pass through on the way to the Summit.
This place is so immense on the dual pass that it is staggering.
It may not be the best in many peoples books but today it was as good as it can get.... unless..... Snowing tonite, 2-6" expected.