I have skiied a lot of different places, but I still think that Vail, Colorado, is the best in the world. It's hard to beat the combination of great terrain, snow conditions, lift system, village, and access via Eagle airport.
In North America, Whistler/Blackcomb, Canada, is a close runner-up. The conditions occasionally suffer due to the lower altitude (I have seen rain at the base, which really sucks), but it tends to be less crowded than Vail. It has one of the best lift systems around, and the drive from Vancouver alone is gorgeous enough to make a trip worthwhile.
In Europe, I have enjoyed Courchevel/Trois Vallees, France, and Lech/Zuers/St.Anton in Austria. I love how you can keep skiing for miles and miles, from village to village. Nothing quite like that in the States. In Austria I had some of my most incredible adventures on skis when I went off-piste with a guide. Also Europe has better food on average than North American ski resorts, both on the mountain and in the villages. The town of Courchevel in particular has an impressive collection of Michelin stars scattered among its top restaurants.
Zermatt and St. Moritz, Switzerland, are also fun, but I was less impressed by their snow conditions and, in particular, their lift infrastructure (i.e., too many T-bars, and long lines full of pushy people). However, the scenery is incredible (the Matterhorn at Zermatt, for example), there's always good hearty Swiss food, and at glamorous St. Moritz you can mingle with the rich and famous.
Common sense is not so common. -Voltaire