I have seen people in this thread recommend Big Sky, Moonlight, Bridger, Big Mountain, Lost Trail, Red Lodge, and Showdown (didnt see that coming)
Having skied most of these I will put some opinions here;
Big Sky - huge, lots of vertical, variety in terrain, used to have a decent park but they are downsizing it this year (no real jumps) It is also super expensive, kind of a touristy gaper trap as well.
Moonlight - on the same mountain as Big Sky, lying on the north facing, windward side meaning its usually in a shadow, gets less snow and is cold. They have a nice park if rails are your thing, heard they are getting some jumps this year but who knows how big they will be. Not a whole lot of variety there unless it gets super filled in.
Bridger - a sick mountain if you dont care about park and are into technical big mountain skiing. Can be real fun if you are creative and there are some great places to ski if you have a beacon (you cant even get on Slaschman's lift without a transceiver)
Big Mountain - a super fun mountain with consistent snowfall, a decent park, and lots of fun stuff to do on the mountain. Whitefish is a cool town too and the people there are super friendly.
Lost Trail - never been there but I have heard lots of good things about it, pretty small though and far from other resorts.
Red Lodge - another mountain I've never been too but most people call it Rock Dodge
Showdown - wasnt really expecting this one, its my home mountain and if you are creative and have some knowledge of the area you can find some real good snow but sometimes this requires taking an old logging road back to the area. I really wouldn't suggest this though.
Out of all those I listed, I think Big Mountain (Whitefish) is your best bet, they have a decent park and a super fun mountain. Its considerably cheaper than Big Sky or Moonlight if you need to buy lodging and tickets are way cheaper than Big Sky.