In my case definitely the top six, probably 8 of the top 10. 3 of the six were heliskiing.
This thread is accurately titled and the key criteria are snow, terrain and low skier density by close to consensus agreement here and also at least paid lip service by the Powder article. Some of the Powder mag choices seem very strange by those 3 criteria though.
Snow is the easiest to quantify, once you dispense with the BS put out by a handful of areas.
Skier visits is a good measure for competition/density, though that data can be very hard to come by in some cases. But for those who ski an area a lot, "we know low skier density when we see it."
Terrain is the most subjective and I suspect leads to the most controversy. Acreage is usually the best simple measure but not always and it is sometimes not measured consistently. And even if it is measured consistently it's a horizontal not vertical measure and skiing is to a great degree about vertical. Again the head-to-head comparison is illuminating. I once again present the case of Jackson Hole vs. Big Sky/Moonlight. The latter reports 2x the acreage but I think there's more powder skiing terrain at Jackson.