I think Jamesj has it mostly right, but I add a few points. The close-to-metro areas (Vail, W/B, Altabird) have most of their liftline/density issues on weekends and are more reasonable midweek. These 3 are among the largest in North America and still offer plenty of powder opportunity because skiers do not distribute themselves optimally. There are plenty of stashes once you know your way around.
For liftlines the Aspen group has the shortest of these 5 by far. For skier density I would say Jackson Hole. The primitive (relative to the others) lift system enforces low density in places like the Hobacks. Alta used to be similar, but with its increased lifts and Snowbird's addition of terrain I think Alta and Snowbird track out about equally now.
My 2 best powder days this season are somewhat illustrative. Snowbird Monday March 13 was 25K, 15K of powder. The morning was a rapid succession of powder laps in Mineral Basin, eventually alternating with the slower Little Cloud when Road to Provo opened. By lunch I was pretty beat but still managed a few interesting shots in the afternoon in the more out-of-the-way places. Jackson Hole Sunday January 29 was 31K, 12K of powder. So much of the vertical is not powder but just getting from one lift to another. But the the powder once I got there was more spread out through the day due to the slower accessibility.