I think there are now about a hundred breweries in this Great Beer State, so this could be a long thread!
1. I love Bell's, of course, and their Two Hearted Ale and Expedition Stout are the gold standards of their respective style categories.
2. Founder's is a great place to visit and they are amazing brewers! Centennial IPA, KBS, Dirty Bastard, the list goes on and on...
3. Shorts is also a great brewpub to visit. They have Humalupalicious IPA and Soft Parade. 'nuff said.
4. Dark Horse is probably the most interesting and enjoyable of all the brewpubs and never gets old. Crooked Tree rocks!
5. I really could go on and on, here, but to finish up with only one more great brewer, I'll choose...Walldorff Brewery! Cobain's Double Dark IPA is amazing!
There are many, many more to talk about from Arobor Brewing (Buzzsaw IPA!), to Black Forest's Pirate's Porter and Dragonmead's Final Absolution, Michigan Brewer's rock the mashtun!