The online Crossfire map has a very good sniping location. The spot can only be reached when you are playing the old school free 4 all section. Notice the building at the very end of the map on the opposite side of the bus. You can get to the very top of the building that is around the corner. First, get to the second story where the chair is found. Get on the top of the chair facing the wall and jump. You may have to run and jump, but eventually you will catch an edge. You can then work your way up to the perfect sniping position.
On the Marine's side (top of the hill), go into the sniping building (the back building that everyone goes to). Go to the very end. Climb up the ladder, and at the top, stop, turn around, and there will be two blue balconies. Sprint jump onto one. You can climb into either of them. From here you can snipe nearly all of the right-hand side.
In the destroyed Bistro at the bottom of the hill, stand on the counter (ground floor). You can look out in between the wall and the roof. From here you can snipe the Marine's buildings.