The classiest hotel in town is the Hampton Inn (3499 Harrison Ave., 406/494-2250 or 800/426-7866, www.buttehamptoninn.com [1], $105 and up), with fancy linens, extracomfy beds, and an indoor pool.
Best Western Butte Plaza Inn (2900 Harrison Ave., 406/494-3500 or 800/543-5814, $125 and up) offers an indoor pool, exercise room, sauna, steam room, and hot tub. The hotel is near shopping, and there’s a complimentary breakfast buffet daily.
Removed from both uptown and I-90 is 
Toad Hall Manor B&B (1 Green Ln., 406/494-2625 or 866/443-8623, www.toadhallmanor.com [2], $99–165), which takes its name and its aura from The Wind in the Willows. The inn (it’s a cross between a small hotel and a B&B) overlooks the country club golf course and is as elegant as Butte [3] gets these days.
If you don’t mind staying a few miles out of town, consider making Fairmont Hot Springs [4] your base.
Links:
[1] http://www.buttehamptoninn.com
[2] http://www.toadhallmanor.com
[3] http://www.moon.com/destinations/montana/butte-helena-and-southwestern-montana/butte
[4] http://www.moon.com/destinations/montana/butte-helena-and-southwestern-montana/the-upper-clark-fork-river-valley/fairmont-hot-springs