Any/all of Summit County in CO, Winter Park CO, Park City/Canyons in UT.
Someone else recently asked about Keystone (in Summit Co) and here were some comments I had:
I stayed there for most of a week this past season. Otherwise, never been before, so here is tourist's impression:
River Run is a nice, condo-centric base area about 80 miles from Denver Int'l Airport via good roads. Appeared to have much of the newest development at Keystone. I stayed at pleasing early 21st century condo called The Springs, couple hundred yards from the slopes. Best thing I liked at River Run was the convenience of riding two back to back gondolas 1st thing in AM to quickly access furthest reaches of resort and then ski my way back to condo for lunch or end of day. There was also a very long crowd eating HSQ that paralleled the 1st gondola for even faster uphill travel out of River Run base. The base is at about 9300' so you're likely to get pretty decent conditions at almost any time you visit during ski season.
I ate in my condo the whole time with family, so did not experience much of the small covey of shops, restaurants and couple bars. They could keep you busy for a few nights, then I think you'll need to go to Frisco (5 miles) or Breckenridge (about 25 minutes) to get more of that. There is a big ice skating area at River Run. For a "fourth time skier" the lifts out of River Run serve some real fine beginner-intermediate terrain on the front face of Keystone (Dercum Mtn), some of the easy runs go for 3 miles. If you are there on lower-crowd weekdays or fringe season I think you'll really enjoy that terrain. Schoolmarm trail features some gorgeous scenery with slopes 200 yrds wide in some spots. The other parts of the ski area have more difficult terrain, but it seemed like there is a nice intermediate trail from the top of almost every lift in the resort.