From what I can gather, there isn't much around here, unfortunately, though I haven't experienced all the area hills myself. I went to Sunburst for the first time last winter, and while I had fun, it was my first time on a snowboard in 6 years, so I wasn't hitting big rails or kickers or anything (not that they really had anything big anyway). I was mostly just getting used to being on the board again and carving the hills.
I used to go much more frequently when I was in high school in South-Central WI, which also doesn't have much, but Tyrol Basin was close enough and has the best terrain park I've been to within an hour drive from my hometown. It's probably about an hour and 45 minutes from Milwaukee, but if you're driving that much then you're better off adding a half-hour or so and going to Devil's Head or Cascade in the Baraboo area. I've always been partial to Devil's Head, as they (used to, at least) have good Tuesday/Thursday evening deals and nice, long runs. Granite Peak up in Wausau is the best hill I've been to in WI, and the UP has some nice hills.
Somebody else would have to give you a better run-down on The Rock, Little Switz, etc. (which the other poster touched on). Personally, I think Alpine is the best area hill, but again, I'm not hitting big stuff, and it's been 7-8 years since I've been there, and a lot can change with management and whatnot in that time.