Law enforcement is most stable, because government almost never shrinks and even if the flying gets reduced you probably still have a cop job.

Military is next, as long you stay very competitive to avoid to avoid being booted out when they get overmanned (they go by performance, not seniority).

135/91 stuff is very cyclical with the economy but has an advantage in that your experience is portable...if you switch jobs, you usually stay in the same compensation range. At the airlines, you start all over at the bottom if you switch.