(See sniper spots below map)  

In my opinion, the Facility is the best level in the game, both in Mission and Multiplayer. It has different colored and textured walls, making it easy to tell where you and each of your opponents are.

One effective spot is in the washroom; there is access to the best weapon, and it's hard to be caught off guard. You can use the pillar in the middle of the room as cover, and if someone gets killed and ends up in the ventilation shaft, they are an easy kill. The major downside to this location is that you can't easily escape to grab body armor without being hit.

Another decent spot is the small room with the glass. If you have a weapon that can shoot through glass, you can take surprise shots at opponents. You can cover two rooms from here, along with the door coming from the tank rooms. You will also be able to notice if someone is sneaking through the noisy sliding door behind you. This tactic is usually noticed quickly, and opponents won't be as easily surprised.