best burger I've ever had is at HopCat in Grand Rapids. Also known as the 3rd best beer bar in the country, HopCat makes a serious burger. Their crack fries are an excellent pairing, and you can't visit this place without trying one of their dozens of drafts from around the world. They are a brewpub as well, and make delicious ales and lagers.
I've not yet tried it, but my inlaws say the best burger they've ever eaten is at a place called Redamaks in New Buffalo, near the Indiana border in SW Michigan. I guess they are pretty well known for their burger in those parts.