With a starting elevation of over 9,000 feet, Northside at Taos Ski Valley is true alpine riding. For a challenge, climb up "Bull of the Woods" to "First Mother" to "Second Mother" to "Big Daddy" and from there, get on Frazier Mountain Road for a difficult fire road climb up to 12,100 feet and the summit of Frazier Mountain.
The views of the Taos valley and Wheeler Mountain are extraordinary, but prepare to be gasping for air for the last half mile of the climb. It's worth spending a little bit of time at the top, soaking in the views, striking an epic pose or two and maybe even seeing a mountain goat. The descent off the top is loose, technical, and heaps of fun. If you're not feeling a 2,000 ft + climb, spend some time exploring the trails closer to the parking lot. "Steam Engine," "Saw Mill" and "Bull of the Woods" are all great fun and offer a suburb taste of Northern New Mexico alpine riding.
Northside is privately owned, so it is important to buy an access permit ($10/day) either at the trailhead or at a local bike shop, and if you're spending over a few weeks in Taos it might be worth it to purchase a season pass for only $50. You can buy passes at Taos Cyclery, Gearing Up Bicycle Shop, or at the trailhead kiosk.