Of your choices, I guess Cannon is your best bet. Especially if you happen to be lucky enough to hit it when Mittersill is open, but that can be pretty rare at times. Besides that, those are all pretty mediocre mountains if you're looking for steeps and glades. Loon and Sunapee have good snowmaking and groomers. I can't really speak for Gunstock and Ragged.
Honestly, if you're looking for trees and more challenging terrain in New England, look at places like Jay Peak, Mad River Glen Smugglers Notch, Stowe, Sugarbush, Sugarloaf, Saddleback. These mountains all have some very good, challenging terrain and generally get more natural snow than central NH.
I'm very new to glade skiing, so I don't know how useful this will be to you, but I can talk about Ragged. I was there just over a week ago after we got a foot of snow.
Reggae Glades is nothing to get excited about, is the type where you have one tree every 20 feet. I did have fun in Moose Alley and Exhibition Glades though. I did not go into The Ravine, but they do seem to have a lot of trails in there. I did speak to one couple who went in there, and the lady said at one point she removed her skis and walked down a bit due to the steepness.
I'd say Cannon and Ragged Mountain have the best glades in this bunch. Gunstock would be the less challenging mountain. Ragged will likely be the less crowded option on most days. Loon and Sunapee get the prize for most crowded. Loon probably the largest resort and Gunstock the smallest.
Cannon is the only mountain on this list that is far north enough to have the climate for consistently good glades. All the others are pretty far south, and while this year has been good to many of them, typically the further north areas not on the list will have better glades and present more of a challenge. Jay, Stowe, Smuggs, Sugarbush, MRG. Those are your mountains for glades in New England. Northern New York is not an area I have a great deal of experience with, so somebody else will have to comment on those.
Cannon is rocky as hell though. I imagine it takes quite a lot of snow to make the glades work.