The Massasauga, just like we have here in Wisconsin. It's just in the far SW of the state, in part of the driftless, so I'd expect it's only far south for Michigan as well. It's a small rattler.
It's why I moved far north, when I came back to the midwest. There have been way too many encounters with with rattlers and copperheads, in my post-midwest life. I mostly just see garter and gopher snakes here, but I guess there are some pretty huge pine snakes.