A concrete apron. We had an aircraft overnighting at one FBO we had previously been to. However, due to some some 'warm' days, when the crew arrived to prep for departure it had sunk about 6 inches into the tar. Moving a Challenger out of the depressions was no 5 minute job.
edit to add: Ramp security, not just from landside but also airside. Ramp rash from dip$hits bimbling around the apron is not welcome.