Northern Vermont:
Stowe, Sugarbush, Smugglers Notch, Jay Peak, Bolton Valley
Southern Vermont:
Killington, Mount Snow, Okemo, Stratton
Like the post above says, most of these are within an hour of Burlington. Each of these mountains, at least in Northern VT, fit different riding types. I don't know too much with any of the Southern resorts, but there are a lot of people here who do.
earthsurfing:Depends on what you're looking for... VT has the most, so that gives you lots of options to explore on different days and in different conditions. Ie. best park, best pipe, best trees, best backcountry access, night riding are all different hills all within an hour of each other outside Burlington.
That's weird as I remember Loon and Waterville going 1,2 in those categories last year.