I find the best glades are the ones which get the most snow or the ones that stay untouched long enough to enjoy the snow which fell a while ago. If the resort allows it I spend almost all my time in the glades. I am certain there are great glades on every mountain, finding them is part of the fun although it is always nice if a local will help show you the way. Below I list a few of the mountains I have skied regularly, by no means is this list exhaustive, and I cannot wait to explore the trees at Jay, Sugarbush, and Okemo as mentioned above.
My Best advice: Throw yourself into the trees even if it does not look like a trail and do not be afraid to fall. I see "lines" everywhere.
MRG - I, and I would think many east coast glade skiers, consider these pines the elite. The slogan "Ski it if you can" holds true on a good portion of the mountain, "Paradise" Amazing thick, steep glades is in fact paradise but only for those ready to Venture. However even the novice and intermediate tree skiers can find and stay in the woods at this legendary mountain. There are plenty of opportunities for the new tree skier to find solace in the gazelle glades and lower glades, and when one is ready, you can find a new state of mind on an early morning run from the top of Single Chair.
Killington - has some excellent glades, for the beginning glade skier try the trails from the top of "Ramshead", "Sunrise" has a few short marked glades but also has some excellent stashes for the wandering skier. be wary of the wider/ marked glades if not hit early in the morning or during a dump they can get icy/slick/ and crusty. Once you venture out on these peaks "Bear" has some marked glades which will get the adrenaline flowing.
New Hampshire - If there is snow...GO!
Cannon - Known for their steeps and wind blown icy chutes you wouldn't necessarily expect to find good tree skiing here. But with the opening last year of the Mittersill double, which basically added an entirely new mountain worth of terrain. This area was an old resort it its own right which went out of business (back in the 90's?) and has since been growing. With Snow the terrain is awesome. I skied it many a day and found myself hitchhiking quite a few times having missed the cutback. With the double now open skiing this area has become a bit easier.
Attitash/Bretton/Wildcat - All of these hills with a good snow cover, one can have a good amount of fun in the woods.
Belleayre - Be willing to hike to get in. be willing to hike to get out. Cathedral Brook all the way to the skiers right has some wide open amazing trees. Go past the AT lean to and just have some fun.
The LOAF (Sugarloaf) - Plan a weeks trip here. Do as much research as you can ahead of time and explore, explore, explore. The locals are not that helpful but buy them a shot or a brew and they will let you in on some of their favorite stashes. This place is huge. and getting even bigger with their 2020 expansion plan. The entrances to these gems can be hidden and hard to get into. DO NOT let that scare you. Often times the best trees are the ones where you stare at the entrance and go WTF. This mountain is far but cheap and Epic!
Tremblant - I grew up in these trees and I would be amiss if I did not mention them. This is where I fell in love with the meditative powers of the trees. If you know where to ski, at what time of the day - you win. Hint: Follow the Sun. Great bumpy glades on the Soleil and on the North Side, but the best stashes can be found on the Edge. This is a great place for a family trip or to go up with folks who do not ski as you have the entire village below, while amazing skiing for you above.