Don't believe the hype on Killington. Stowe, Jay, Sugarbush -- all good.
Killington is privately owned and maintenance on the facilities is low at best. They way exaggerate their trail count AND their vert, which includes a completely un-ridable green which literally crosses a parking lot before turning into a flat snowshoe trail for nearly a mile down to the Skyeship base. Last time I went we were ready early, cranked up for first tracks in nearly a foot of pow which had been falling all night. We were second in line for the gondola, which made it halfway up the mountain and then stopped, trapping us for 25 minutes as we watched everyone else track up that beautiful snow beneath us.
There are tons of flat areas and traverses (which, as other posters said, are counted as "trails"). And the "six mountains" stuff is pure crap...it's one mountain with six bumps on top, which have different names -- Sugarbush and Stowe are both steeper and bigger and each actually have two distinct mountains accessed with one lift ticket. What Killy IS famous for is night life. It is a party town.
Jay Peak would be my fave if it weren't so far away. Going to Jay is like being in a different country. Most of the people speak French. If the tram is down, it's not really a big deal because it really only accesses some insane trees and two blue trails connecting to all the stuff you can get to via the lift which lands you just below the peak. That said, the top is sick. Almost scary...you step out of that tram onto a very small landing area that barely holds all the people on the tram, with a 1500' drop into the woods right in front of you, and only two driveway-wide trails, one of which I've never seen actually open. But it's a hell of an experience! Closest thing to out-west style riding in New England.
Sugarbush is pretty intimidating when you first drive up to it. It hardly looks like there are any trails, the mountain is so big and they're so far apart (and narrow). Lincoln Peak is the main mountain, and it's BIG. Mt. Ellen is a five minute drive down the road. If you're really good, you can access one from the other via long backcountry trip through the valley between the two mountains. But you had better really like trees. There is a gondola between them, but I've never seen it open.
Stowe is just epic. My favorite. The five double-black "signature" trails on the face of Mt. Mansfield are some of the steepest shit in the Northeast. Stowe doesn't have a huge trail count, but the trails it does have are loooong and fast. A run from the peak to the Toll House area is nearly 3 1/2 miles, though it isn't the most challenging thing you'll ever ride. The wind can be an issue -- in fact, the first black run I ever did was at Stowe, accidentally, when I literally got blown onto the entrance to Hayride. Across the road -- accessible via a gondola, is Spruce Peak. They market it as the more family-friendly mountain, hoping to keep the small kids off of Mt. Mansfield. It's not nearly as steep, only has two short black runs, and lots of bunny-slope area at the base where you'll have to weave through a thousand ducklings in ski school. And one of the best "resort" towns in Vermont -- lots of restaurants and bars.
Stratton has a lot of terrain, and is pretty fun, but it's spendy and has a kind of snotty attitude - probably because its so damned expensive. Lots of pretty people in never-been-worn-before gear. They have a village, but aren't really close to any town, so unless you're staying there ($$$), you're pretty much out of luck for night life. Bromley is actually good for a smaller mountain. It faces Stratton, and is usually sunny when Stratton is covered in clouds. Mt. Snow is fun, but nothing real challenging. Great parks, if you're into that. I hate Okemo (though not nearly as much as Killington). It's always crowded, and the lift system sucks for navigating your way around the mountain.