Of places I've been, Snowbird and Squaw Valley had the most "holy crap I would die if I tried to do that"-type terrain.
Alta had some nasty-looking stuff. Some of the hike-to stuff at A-basin looked pretty experty. There was definitely a chunk of stuff at Aspen Snowmass that I couldn't get near. Vail had a surprising amount of tough-looking terrain, although also many thousands of acres of (relatively) tamer runs and bowls.
I haven't been to either (yet), but Jackson Hole and Whistler also tend to be high on these sorts of lists. I'm pretty sure Jackson is #1 in Ski Mag's ratings for terrain challenge, FWIW.
Chamonix kills more people annually than anywhere else, so I'd guess the skiing can be tough. Watch out for those crevasses...
In the Eastern US, the Front Four at Stowe are damn tough trails (especially when conditions are sketchy), but it's not a whole lot of terrain. Haven't made it to Mad River Glen yet, but they're supposed to have quite a bit of very difficult terrain, and of course they don't groom much.