RS Pro DSO entry
A good multimeter will go a long way to diagnosing most problems. In the field, it's usually a failed connector or a broken wire. Once the stability of the power supply has been established and some basic continuity checks have been performed, it's time to move onto something more technical.
In the lab environment, at the build stage, my scope has been invaluable. Although analogue, 1980's technology, it proved its worth many times. Now I'm often called upon to diagnose comms problems though, I find I have an increasing need to store and analyse waveforms.