Hmm... all mine have been electrocutions. I don't know which was the most painful.
*Working on a camera. I wasn't aware that the flash had its own capacitor - I thought I was safe enough by just taking out the batteries. I don't know which part of the circuit I touched, but that sucker discharged across my thumb.
*Working on an LCD display. I had an LCD with a burnt out power supply/inverter board. I couldn't find a replacement, so I figured I'd just modify one from another model. Anyway, I'd wired part of the new one up and wanted to test my work, so I plugged the LCD into the wall. I forget why, but I wasn't satisfied with how the AC input sat, so I proceeded to grab it, forgetting that the damn thing was still plugged in.
*Last one was working on my car. I had a couple of dead cylinders, and was working to eliminate causes. I was checking the spark plugs, and FWIW my car has a high-energy ignition. Not having a test light, I did it the old-fashioned way by pulling the spark plug (still attached to the ignition module), setting it on top of the plenum, and starting the engine. Well, the ignition was not the problem, the spark plugs were fine. But the spark plug I had pulled out started to roll off the plenum. Not wanting to fry electronics in my engine bay, I proceeded to grab it. Zap! Zap! Zap! Zap! My whole arm was convulsing to the rhythm of that thing, which made it difficult to drop the spark plug.