For quality tree riding Winterpark or Steamboat are going to be your spots. The other great tree places would be Vail or Beaver Creek.
Winterpark's trees are the best in the state imo, if you like expert tree riding. Lot's of tight squeezes and quick turns. Lot's of good powder.
Steamboat probably holds the overall title. Tons of tree runs that are easy to figure out and fun. The South side of the mountain is mostly Aspens while the North side has plenty of evergreen runs.
Summit has ok tree riding but I have never found it to be of the quality of the above mentioned places.