I've skied all of the places mentioned with beginners (except for Eldora) and would definitely say that Copper best fits the bill. I'm a huge Aspen fan-boy and definitely think that Buttermilk and Snowmass have lots of great terrain for your group, but Copper would overall be easier, especially if you can find a place on the west side of the village.
Stay away from Winter Park, in my opinion. Most of the greens are cat tracks which cross other more challenging runs which can create confusion and some scary moments...(or they are way too flat and involve more poling/skating than you will find anywhere else)
Stay away from Winter Park, in my opinion. Most of the greens are cat tracks which cross other more challenging runs which can create confusion and some scary moments...(or they are way too flat and involve more poling/skating than you will find anywhere else)