That's funny about saying sheriff is highest law enforcement agency.
It is depends on situation and jurisdiction.
If you are in city jurisdiction like NYC so NYPD is LEO, but they could assisting with federal LEO if it is necessary.
If you live in unincorporated area so sheriff is LEO, but in bigger counties like Fairfax County in NoVA has own county police department.
If you are in state parks or interstate highway (in Virginia) so state troopers are LEO in those area, but if there no police department or sheriff department so it means state troopers will be responsible for patrolling or contract with other counties for LEO.
If you are in federal jurisdiction so FBI, US Marshal, DEA, ATF, ICE, CBP are patrolling, depends on area.
Federal > State > County > City