I don't have a list to find the highest, but I suspect that Spektr-R RadioAstron (used for long baseline radio interferometry) is one of the highest altitudes that isn't associated with a Lagrange orbit.
The Moon interacts with its orbit, so the apogee changes over time. According to the User Handbook
the apogee distance will vary from 286,938 to 371,233 km
It has a very high eccentricity, so I believe there are others with a greater semi-major axis. I'm not sure which measure the question might want for "farthest out"
