The Polderbaan (18R/36L) west side.
Note: Beware of tightened parking restrictions all around the Polderbaan area! Police strictly enforce them!
The Polderbaan is the newest runway at Schiphol. The runway is in the middle of nowhere (by Dutch standards anyway) and is therefor used heavily as only few people are hindered by noise. Therefor a lot of interesting planes can be seen from here. Best yet is that the viewing possibilities are excellent. On the west side of the runway you can walk along the entire perimeter of the runway with little blocking your view. On the east side of the runway the view is a bit more restricted, but still very good.
The west side of the runway is in particular usefull in the afternoon. The later the better really. Early morning the sun will be facing you. Late afternoon, in particular in the summer, the sun will be in the back.
You can walk along the pretty much the entire runway, as there is a bicycle path running alongside it. Therefor you can pick your preferred spot. A side-on shot of a 747 required only 50mm or thereabouts. So even with a simple throwaway camera you should be able to get some nice shots here!
With the exception of one crash gate roughly halfway down the runway as well as the ILS beacons there are no obstacles worth mentioning. Though the plants besides the runway could use some trimming.
It is rather far from Schiphol and there is no shelter in case of rain. During the weekend sometimes there is a mobile snack car.
You can get here by public transport, but only if you do not mind to walk for about 3km. To do so take busline 300 ("Zuidtangent") and stop at busstop "Vijfhuizen". On the map to your right it would be on the top left corner.
From here walk back a bit and take the first road on your left into the road called "Vesting". Walk on till you are on the roundabout, cross it, and at the end of the road go to the right. Walk on, and cross the main road (N205). Continue walking straight on for another kilomer or so. You will then be at the intersection pictured to the right where you can choose which spot to go. It is the only intersection so you can't miss it. I would prefer going to the left here, but it is up to you to decide which you prefer.
Did I mention that you can walk down most of the 4km length of runway? So if you still got some energy left, you can walk even more!
Fortunately getting here is much easier by car.
From Schiphol plaza you should get onto the A4/E19 motorway in the direction of Den Haag. Exit at the first exit (Hoofddorp / Aalsmeer). At the end of the exit turn to the left onto the N201 (Kruisweg, changes halfway into “Weg om de Noord”) in the direction of Hoofddorp.
After a while you will be at a large intersection with traffic lights. On your right at the other side of the intersection you will see a fire station. At this intersection you must go to the right. At the second intersection you will have to go to the right. (if you drive on you will end up in a dead-end). The road you are now on is the IJweg. Continue following this road to the end. Once there you will be at the spot.
It is very important to park ONLY at the official parking area. Do NOT park at the roundabouts at the end of the road. The parking restrictions have recently been tightened, and the police are enforcing the new restrictions tightly! A parking ticket will set you back roughly 60 euro.