Keystone and Breck are considered to have the best Terrain parks around. And imo Keystone has WAYYY better riding then Breck does if you enjoy trees and such. Breck has some fun rollers that you can either bomb or to 180's and 360's off of.
So if you like trees, go to Key, if you like groomers, go to Breck. The park at both spots are top notch. I'm not much into park riding at this time, otherwise I could reflect more on them. But I can say it seems that people like the park more at Breck from what I've seen.
I ride park about 50% of the day and mountain the other 50%
keytone have a good park, dont really go to breck much, it too windy,
as for keystone, it has some good stuff on the backside,
nothing like vail for trees, actually vail is the best for trees. near summit county.
If you want a disorganized cluster fuck skate board style park go to Keystones park. It's a stop and go park with very little flow. They do offer unique jibs which can be serious fun, the jumps aren't for everyone.
Breck is more calculated and thought out with its lines and follows a more standardized format of jibs and jumps. Also their pipe blows Keystones out of the water.
Plus to Keystone is faster park chair than Brecks plus you're away from a major base area so you don't get the annoyance of families like you do at Breck.
If you're looking for better free riding Breck blows Keystone out of the water. The frontside of Keystone is land of the patrol nazi douche types. It's also crowded as balls and the run lay out funnels everyone into the greatest clusterfuck on the slopes. The backside is good occasionally and only if you know which lifts to get you to certain areas. The trees have a lot of dead fall and if you're not used to tight trees with crazy obstacles sucks balls.
Breck while it does get wind isn't that bad if you know the areas to go to and which valleys to get into. Get down in the valley between peaks 8 and 9 and you're golden. Peak 10 can be fun and if you want rollers to fuck around on shoot over to peak 7. On a pow day you can get some good stuff off of 6 chair and others in that vacinity.
Now if you want a mountain with it all in Summit County go to Copper. The jumps are some of my favorite to hit in the whole county and the flow is solid, plus you can get a 40 minute park lap in if you want. The jibs are a bit lacking and often crooked, but the pipe is solid. Also the free riding is solid you can really explore around and find stuff.