First, think about your mountain bike style. Are you planning on riding a mix of climbs and descents? A trail bike is a good option. If you want to ride quickly and climb easily, look at efficient, low-weight cross-country mountain bikes. Or perhaps you're looking for traction for sand or snow: get a fat bike.
Mountain bikes with 29-inch wheels or "29ers" have become increasingly popular with off-road cyclists. The 29-inch wheel is larger and heavier, making it slower to accelerate yet offering the best momentum. Frames and suspensions are designed to accommodate a bike's wheel size. Shop all 29er mountain bikes.