At KDTW, there is a parking structure at the South Terminal. Go all the way to the top. On one side, you get a great view of the gate area, and the other affords a commanding view of 3L/21R and 3R/21L action. Great place to take the scanner! They chase you away if you are on any of the perimeter roads, so the parking structure is the best place to go! You pay by the hour to watch airplanes, but they can't chase you away!
Below is the Jepp for KDTW.
But at the same time, I agree with rekno13, best place to be is right under!
Best place in the world to watch airplanes is the beach at Philipsburg / St. Maarten - Princess Juliana (SXM / TNCM). See Below!