Which equipment for electronics should I always have on hand?
Digital multimeter
Soldering iron
Oscilloscope
Power supply


If you've got those 4, you're in decent shape.

After that, I'd add a frequency generator, then a spectrum analyser.

If I were starting an EE lab at a company, and I was going to buy those tools, here's what I'd start with:


Fluke 117 multimeter ($160)
Weller WES51 soldering iron ($88)
Tektronix 1001B scope ($880)
Agilent E3631A power supply ($1383)


If I were a hobbyist just getting into the field, I'd limit myself to the Tools section of Sparkfun, plus the Tektronix scope.