Haven't ridden FOMBA yet, but hear great things. Ditto Bear Brook State Park in Allentown.
Up north, you can find some good rides, including lift-served stuff if you swing that way (
Waterville Valley has some, as does Bretton Woods;
Highlands is a full-on mountain bike park).
If you want to earn your burn in the Waterville Valley region, you can find a variety of rides from forest roads (Livermore Road, Old Waterville Road, Tri-Town trails) to singletrack (Waterville Valley Resort fee trails, local trail network in Thornton). You can piece together a good loop (I hear) with the Dickey Notch Trail and several others... you might want to stop by Rhino Bikes in Plymouth for advice.
Given my proximity, I am partial to the
Waterville Valley trail network, and would recommend rides like Livermore Road to Upper Snow's Mountain (moderate to challenging climb, with a few technical patches) or to Lower Snow's (fee trail -- medium technical challenge on mixed track). You can ride the Greeley Ponds Trail most of the way to the Ponds if you're good, though it can be rough in places (rock gardens and a gnarly rooty section). If you like a steep climb, go Greeley Ponds to Timber Camp Trail. In the north end of the fee trail network, the Osceola - Moose Run - Wicked Easy loop is fun and moderately challenging (as a friend says, he feels like he's riding in an Escher drawing... always going up), or you can kick it up a notch or two and go Osceola to John Deere, etc. In the South End of the fee trail network, Snow's Brook to Drake's Brook is a fun loop on mostly-fire-road terrain.
PM me if you want more specifics.
