Sun Valley is a world-class ski resort in the mountains of central Idaho, built around Bald Mountain — known locally as Baldy. It was America's first destination ski resort, opened in 1936 by Union Pacific Railroad, and it has stayed at the top of the pile. The resort attracts serious skiers rather than the apres-ski crowd; the focus is on the mountain itself.
Baldy is legitimately excellent. The vertical drop is over 3,400 feet, the runs are sustained rather than rolling, and the lift system is efficient enough that lines are rarely a problem even on busy days. Runs like International, Fire Trail, Easter Bowl and Upper River Run give advanced skiers plenty to work with. February and March are typically the best months: the holiday rush has cleared and conditions are usually at their peak.
The nearby town of Ketchum is the practical base, with restaurants, shops, and most of the accommodation. Sun Valley itself is the resort village. The surrounding Wood River Valley is worth exploring in summer too — hiking, mountain biking, and fly-fishing on the Big Wood River.