If you want off-piste, Mount Bohemia. There is no competition. IMO it's the only place really worth skiing in the Midwest, and I've skied at many places in the Midwest (Minnesota, Wisconsin, and Michigan). It has about 1k vertical feet and terrain that rivals most Rocky Mountain ski areas, and it gets a lot of lake effect snow. Mostly tree skiing, some open bump runs, and some cliffs. Potential downside is that they don't have grooming equipment so if you like taking easy runs on groomers from time to time you don't have that option.
If you want a mix of on-piste and off-piste, based on my experience, Whitecap is your best bet. Blackjack and Lutsen are great for groomers (by Midwest standards) and both have a little off-piste. Runners up (groomers only) would be Giants Ridge and Spirit.
Places I have not been to but have heard good things about are Granite Peak and Indianhead. Also Porcupine Mountains but if you're going to the UP you'd be an idiot to not ski at Mount Bohemia so it would be pointless to go to Porcupine.
Summary:
Places I have been:
1: Mount Bohemia (no competition at all)
2: Whitecap
3: Blackjack and Lutsen tied
5: Giants Ridge
6: Spirit Mountain
Places I have not been:
Granite Peak
Indian Head