Your electrical tester.
Besides an electrical meter. Having an efficiency tester. Once the system is operating check the condition of burner combustion. All adjustments to the burner should be followed by a smoke, O2,CO,CO2, excess air and a stack temperature. After doing this you can be assured that the burner is operating at its peak efficiency. Less chance of sooting up a house and feeling confident everything is operating to factory specifications.
There is no best. You need it all. Electric meter, combustion anylizer, oil pressure gauge, and smoke kit.