Best places I've skied in the Midwest are Mount Bohemia, Blackjack, and Whitecap. I haven't skied in Illinois, but from what I've heard everything there is small even by Midwest standards.
Mount Bohemia is without question the best place in the Midwest if you're comfortable on advanced-expert terrain. Nothing is groomed. Mostly tree skiing. If you're comfortable on double blacks all day (or the Australian equivilant, I don't know if you have the same rating system there) you'll love it. If not, you won't. Sometimes they have amazing snow, but they have a short ski season and don't make any snow, and I've been rained out of planned trips twice. If possible, it's best to make trips there as a last minute decision.
Blackjack and Whitecap are both very big for Midwest ski areas. After Mount Bohemia, they're my favourite places in the Midwest that I've been to (I've probably skied 20 or so places in the Midwest). Mostly groomers but some ungroomed, especially Whitecap, which has some pretty steep bump runs that would be double black even by mountain standards. I've heard good things about Devil's Head but have never been there. Pretty much anywhere in the Upper Peninsula is probably going to be decent by Midwest standards.
Keep in mind that most Midwest areas won't be much more than a bunny hill by mountain standards. If you can make it out west, do. I'd recommend Utah, but it depends on what you're looking for and how good you are. I love Snowbird (and Alta) but it's a very steep ski area so you won't have much fun there unless you're a pretty proficient skier. Same with Jackson Hole. If you go to Alta, know that they don't allow snowboarders, so if you are with any snowboarders it's out. Avoid the east, the west is better.