First I would like to make a distinction. Are you into hardcore spotting (collecting registration) or aviation photography?
The observation deck at AMS is great for spotting. For aviation photography heat haze can be an issue as the runway is quite a distance away. Good photos can still be had from the observation deck, but it will be more limited to planes taxiing close by. You can however take the bus to several of the runways / taxiways around AMS.
Zurich is great for spotting as well. It has an observation deck as well which is quite good for spotting and photography. In the morning the planes typically land on runway 34. The views from parking garage P3 can't be beat. Disadvantage is that in the afternoon ZRH really slows down.
Another one to consider is DUS. The views are very good. The terraces are close by the runways as well, so photography should be no issue. Only the far runway will have heathaze difficulties. One thing to consider though, DUS used to have a large Air Berlin operation. With them having gone bankrupt, the airport might have slowed down a lot. I have not been to DUS recently so can't tell if it is still busy enough to keep you entertained. There is a very active message board on DUS that also shows expected movements except for the based aircraft.http://www.dusinfo.net/index.php
From the main terminal of FRA you can walk to the eastern side of the airport into the approach path. It's about half an hour's walk I would guess. For planes landing on 25L and 25C this is an ideal spot. But if the wind if from the wrong side, then the spot is not that great. The planes will be too high for nice photos. For collecting registrations you can sit by the windows at the terminal 2 MacDonalds.
MAD is also quite good for photography, weather permitting (heat haze!). You can stand on the hill next to the power cables shown on this google maps streetview. You can overlook several taxiways and runways as well as terminal 4. Again you should count an about a half hour walk.https://firstname.lastname@example.org, ... 312!8i6656