The "Theme building", that's the futuristic-looking (actually 1960) structure on the high angled stilts right in the centre of the airport, among the parking structures. It's between Terminal 2 and Terminal 6, but you can see it from many points of the airport.

Picture of building and airportmap on: http://www.lacvb.com/mod29/mod29welcome.html

Take the elevator to the top. There's a viewing gallery and a pleasant restaurant which never seems too crowded so you can take a window table and while away the time. Choose a table that gives the best views (generally looking eastwards so you can see the approaching aircraft).