Which of the following is the best instrument to use for checking the keying waveform of a CW transmitter?
Why A keying waveform is amplitude plotted against time, so you need an instrument that displays a signal's shape in the time domain. An oscilloscope does exactly that, letting you see the rise and fall times of each dot and dash. Too-fast rise/fall edges produce key clicks (splatter), and the scope trace shows whether the shaping is soft enough to keep the sidebands narrow.
Watch out A field strength meter only shows relative RF level with no time detail, and a sidetone monitor just generates an audio tone for the operator's ear, telling you nothing about the actual transmitted envelope. A wavemeter indicates frequency, not waveshape.
Waveform means shape versus time, and shape versus time means scope.