B&K 747 Tube Tester. This is one of the best tube testers ever made owing largely to the huge number of tubes it can check (96% of everything ever made) and the preset sockets for quick testing of common tubes.
Also, because it is solid state it doesn't need periodic calibration so it is eminently reliable. Checked against a 2nd master B&K 747 and in perfect working order.
If you have a tube amp, knowing your tubes are good is the single best investment you can make.