I picked up a 27.5+ Specialized Fuse and it gets most of the miles. It's not nearly as fast as my 29'er but it's just fun to ride. The 3" wide tires absorb a lot of the little bumps and provide much better traction on the steep, loose stuff.
Go to a bike shop and pedal around on a few and see what you like. Lots of bike shops have demo bikes.
Also, get a good pair of bike shorts and chamois or a liner. I've had good luck with both chamois and gel but I rarely ride more than 3-4 hours.
