Didn't Carl Sagan and other scientist work out that actually, the chance of intelligent life existing on other planets is a lot higher than we think.
In fact in my opinion I think it's incredibly arrogant to believe that we are THE only humanoid type life in the whole of the universe.
Secondly if UFO's do exist and they are being covered up, what's the point? I don't think there would be the mass panic everyone thinks there would be. Also if such life was hostile, I'm pretty sure they would have destroyed us by now, get 'em while their weak after all.
Thirdly if my memory serves me correctly the British MOD have admitted their pilots have experienced flying objects that outpace even our fastest jets and that they have no idea what they were.