The quality of the trails is definitely dependent on how much rain has fallen, but at this time of year I'd look at the following if you're looking for single track:
Evart - some sand to start, but then loamy woods with some technical/rocky climbs. Whoops too .... and more whoops .... About 24 miles.
Cadillac Long Lake - 26 mile loop, likely the tightest trail, with a fair share of whoops, some great elevation and roots, and no rocks. Primitive camping on site, enter from the north until 131 gets fixed.
Grand Traverse - some really good elevation, a little bit of sand, moderate to challenging trail.
There are dozens of other trails that I haven't ridden, but I'd look for those marked 50 inches or less ... damn those ATVs
