I've heard good things about the Bar W near Whitefish.
http://www.thebarw.com/

Although it's been 22 years since we stayed there, Lone Mountain Ranch in Big Sky, Montana, which is not too far from Yellowstone and could easily be done in the same trip, is still going strong it seems. See www.lmranch.com. We loved the place; it was our honeymoon, but we noticed that there were people of all ages enjoying the ranch. We have a 15 year old son now, and this post reminds me that we should take him there before he goes to college. Check it out. Have fun.