AMS has one of the best spectator decks in Europe. You reach it from within the terminal building by elevator and its got everything you need for a good day's avitaion spotting - including Pizza Hut for when the belly needs filled! Most of the aircraft around the terminal parking stands can be easily photographed with a good 75-300 lens.
There are also some great areas at the runway ends but I'll leave the details to the local experts!!