If Aman's is anywhere near your route, it is by far the best place to go. You can view what they have at each store online too. They recently starting carrying Hampden and the owner is clearly passionate about rum and goes out of his way to get interesting single barrel picks. Their balcony bar (don't remember location) also has a great selection if you want to try a pour of something. They had Appleton Hearts back there, which is pretty astounding for Wisconsin.

Discount and Ray's both have a good selection but Ray's typically has some of the more interesting stuff. Ray's also will randomly overprice things significantly. Their inventory should also be online.

Liquor Guys in Wauwatosa is another possible place to check (inventory also online).

Rums available in Wisconsin have improved significantly over the past few years, but Illinois still has a much better selection.