What is the highest EM frequency ever detected or produced?

The highest energy particles around are cosmic ray protons.  They can have energies in the 10s of TeV--several times more energetic than we will be able to produce at LHC--14 TeV center of mass energy collisions.

So these protons probably collide from time to time and produce various things which will include photons which will have energies on the order of at least 10-20 TeV.  On earth such a photon would run into other stuff and produce a big shower of particles.  The Pierre Auger experiment looks for events like this.

If you want to assign a frequency to that energy, divide it by planck's constant--4.14 x 10^-15 eV-s.  So that's going to be on the order of 10^27 or 10^28 hertz.