http://en.wikipedia.org/wiki/Sniper
Top Three:
British Army CoH Craig Harrison of the Household Cavalry successfully engaged two Taliban machine gunners south of Musa Qala in Helmand Province in Afghanistan in November 2009 at a range of 2,475 m (2,707 yd), using a L115A3 Long Range Rifle rifle chambered in .338 Lapua Magnum. These are the longest recorded and confirmed sniper kills in history.[21][22][23][24]
Canadian Corporal Rob Furlong, formerly of the PPCLI (Operation Anaconda, Afghanistan) - achieved a recorded and confirmed sniper kill at 2,430 m (2,657 yd) in 2002 using a .50 caliber (12.7 mm) McMillan TAC-50 rifle.[59]
Canadian Master Corporal Arron Perry, formerly of the PPCLI (Operation Anaconda, Afghanistan) - briefly held the record for the longest-ever recorded and confirmed sniper kill at 2,310 m (2,526 yd) in 2002 after eclipsing US Marine Gunnery Sergeant Carlos Hathcock's previous record established in 1967. Perry used a .50 caliber (12.7 mm) McMillan TAC-50 rifle.[59]