If long Groomers are your only criteria-Mt Snow, Okemo, and Flatton are all about equal-they will all be equally empty on weekdays and all will still have a good crowd on weekends (not mid season crowded, but you'll still see lots of folks). I'd always take Mt. Snow out of those three because it's closer and there is much, much greater variety (the trees and bumps at Mount Snow are still skiing well!).
However, the best ski area of the 4 is Gore. I love the lay out of that place, the mellower vibe, and the far more interesting terrain, and they picked up more snow this month than the other three. For Groomers, I guess the VT places are better (though Gore is decent), for everything else...including the scenery as the 'Dacks have it all over the Greens, I'd go with Gore.
Oh, and from a recent experience if long cruiser-delight on cool trails is your bag, you might want to take a longer drive and check out Wildcat in NH. 2000ft vert of long, uninterupted, scenic, windy groomers all over the Mountain.