Note most of these all require passes to park (metropark vs. state park)
Potowami (Poto) - http://www.michigandnr.com/parksandtrails/Details.aspx?id=229&type=SPTR#map-tab Probably the most popular place to run trails, but also mountain bike in the area. It’s ~25 mins from Westside of Ann Arbor and is rather hilly/technical for the area.
Island Lake - http://www.michigandnr.com/parksandtrails/Details.aspx?id=462&type=SPRK Mostly flat like a pancake with very few hills, this is another popular spot to enjoy the woods, but without the hills to punish. It’s also an incredibly popular MTB spot for beginners.