In Utah: Park City, Canyons, Deer Valley. You don't even need to rent a car. You can take a shuttle bus from SLC airport that gets you to Summit County in 40 min, and from most hotel locations you can take a local free bus every morning that runs every 15-20 min and goes to all main Summit county resorts. Park City and Canyons are now connected. Park City Main St has a really good apre-ski vibe and atmosphere.
If you rent a car and go a bit further, you can explore awesome resorts of Big Cottonwood Canyons (Solitude and Brighton- small, local, cozy, my favorite resorts) and small Cottonwood Canyons (Snowbird and Alta - large and vast with tons of challenging runs; favorites of many advanced skiers, not my personal choice though)
CO is overpriced and the quality of snow is not as good as Utah's (Utah's snow is dryer, thus more powdery), but if you want to experience huge and vast resorts with high elevation, great infrastructure, and numerous runs, check out Vail. Traffic on CO's I-70 can be very bad though. Give yourself a few hrs to get back to the airport.
Hope this helps
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