Too many to pick one, here's a top five of my daily drivers:
1. Pace MBT solder/desolder station
2. Fluke 45 Dual Display Multimeter
3. GenRad W10MT3A Metered Variac
4. TV-7/U Military tube tester
5. Amber 5500 Programmable Distortion & Noise Measurement System (poor man's AP)