Depends on a number of things but as a rule:
Local bimble at 2700ft QNH, or 2300 if the cloud is a little low. Keeps it simple if I'm doing a route via, say two or three waypoints. These numbers keep me away from the many people at 2000, 2500 and 3000. Cloud lower than that, probably won't go unless I'm just circuit bashing for practice.
Cross-country, cloud permitting I'll find a low quadrantal (eg. FL35-FL60) but this obviously depends if I'm transitting CAS - and whether FL35 is available. In class G, a low FL will keep me above most ATZs, glider launching sites and some restricted or danger areas (eg. prisons and gas venting stations).
All my flying is VFR.