I thought this one might be fun, since everyone here is an AvGeek, and so I thought the most underrated locations would make for an interesting thread.
I am not sure if you and me share the same definition of underrated. If you had named your list "top 10 best known spots" the list would no be all that different.
Some of the spots I would say are actually overrated. For instance the Manchester viewing park. It faces the sun most of the time, but it still attracts a large crowd. Arguably the mounds at the other side of the runway offer a much better view.
For plane spotting (collecting numbers) the Polderbaan at AMS is not recommendable as you are oretty much guaranteed to miss almost half the movements. For spotting the observation terrace can not be beat. For photography the polderbaan is ok, although here to the sun will be in front of you most of the time (assuming you go to the official area). But because it is well known I would not call it underrated. The area around the Aalsmeerderbaan however I would call underrated.
As for real underrated airports I would think LaGuardia fits the bill. Most spotters visiting NYC go no further then JFK and/or EWR. However, LGA offers great bank shots near the Metz stadium. But because there are no widebodies and traffic is restricted to USA and Canada this place is often overlooked.
Bonus: Worst- ?..DTW is the other airport that comes to mind, with dots in the windows (WHY?)
Another airport that is underrated. Unless this has changed since my last visit a few years ago the windows facing north at the narrow side of the terminal do not have the dreaded dots.