To me the most telling evidence is the actual records from the time period. There are numerous classified messages, memorandum, and reports by the military discussing UFOs and what they were/weren't/might be. All were written by people who should have known something if there were really an alien spaceship that crashed in 1947.
All of the officers who wrote these reports had no idea that what they were writing could later be read by the general public (there was no FOIA at the time) and were expressing honest opinions. The head officer at project SIGN (the first AF investigation into the matter of UFOs in 1948) wrote that they desperately wanted to find a crashed UFO because they had little idea of what the source of the UFO reports were. Meanwhile, the General staff was writing letters to project SIGN demanding that the group determine what they were!
If there were a crashed UFO stashed away, one would think that somewhere, somebody would mention something about such an incident. At least you would not have the head officers in the USAF demanding an answer to what these UFOs were and they certainly would not have wasted a lot of time and manpower creating a group to determine the source of UFOs when they already knew the answer to the question one year before!