The single best all purpose sniper rifle is the M24. It has good range, is bolt action for increased accuracy, and allows for a fairly decent rate of fire. If you could only use one sniper rifle, learn to use the M24. The choices vary from the M24, based on range or speed. A semi-automatic sniper rifle will put out more volume, at the expense of accuracy. The large bore .50 caliber rifles such as the Barrett increase range considerably, but have a very loud sound, and are easy to hear.
The optimal range for a sniper rifle is 600-800 yards, and 1200-1500 yards for a large bore rifle. The furthest effective range is 1000 yards for a .308 caliber, and up to 2500 yards for a .50 caliber.