Honestly, all of these mountains can get crowded on weekends.
Killington has some good parks. It's very big (wide) and its been said its not the best for riders. Better nightlife however.
Okemo is mediocre IMO. It's park has been slow to get set up. Lots of terrain but nothing unique. It gets a lot of new yorkers. It can get very icy, moreso than other NE mountains.
Stratton is good, probably a bit more expensive. Good vertical, good parks. Like Mike said, all the trails are kind of the same.
Mount Snow is probably my favorite "big" northeast mountain. Their park, Corinthia, is one of the best you'll find. Good vertical, good mix of terrain so you won't get bored. IMO, Snow does a better job of managing the crowds which results in less time in lift lines.
If you're a weekend warrior or going up for an extended weekend, my advice is to find a motel or B+B within driving distance of a few mountains and try out a bunch. There's a bunch of medium-small mountains near Snow, Okemo and Stratton that are just as good, but with less terrain (i.e. Sunapee, Magic, Bromley).
To answer your original question: Stratton. More vertical.