Depends what kind of trees your after.
If your after some of the best aspen skiing in the world, go to what the locals call "Wally World." This is the area with Tomahawk and Rolex. This is looker's right on the map. Also check out some of the "clock" runs.
If you want some good ol' fashion pines, go for the Storm Peak area. Going up Storm Peak, look left and you will see some quality trees. Also look around Buddy's and dip in.
Another excellent location would be the Pony Express area, but at this moment it's closed.
A great thing about Steamboat though is that there is excellent tree skiing anywhere you look, so don't be afraid to explore.