I'll vote for MAD
as well. I can't vouch for outdoor spotting since I don't usually hire a car when I'm in Europe, but here's the indoor report:
My last visit to MAD
was in December 2005, so I haven't been to the new Terminal 4, but the cafeteria in T2
offers a great view over the arrival runways (as long as you visit after noon when the sun moves behind.) I'm sure the traffic is a little more dispersed now that T4
and the new runways are open, but back in December pretty much everything had to taxi past the terminal. MAD
gets a pretty interesting selection of Latin American airlines that are hard to find elsewhere in Europe - Avianca, LAN, Aerolineas Argentinas, AeroMexico, etc. - so that makes it unique.
I flew to MXP
in February of this year and was pretty disappointed. The terminal has lots of big glass windows and viewing is great - the architect deserves congratulations for designing such a viewer-friendly building. However, the runways are somewhat distant, and it seems like Alitalia gets preferential use of the close-in stands - all the interesting stuff tends to park far away on the remote stands.