Dioxygen difluorine (FOOF) is so reactive (unstable) it decays into O2 and F2 at about 4 % /day at -160 Celsius. It even reacts violently with ice.

Edit: Because everyone liked FOOF, get ready for tetraoxygen difluoride (FOOOOF) which is even more reactive. Apparently reacts explosively with elemental sulfur at -180 Celsius to form SF6.