My #1 is a German army surplus folding pick & shovel. This thing is quite heavy to carry around, but it is also very strong and pretty much indistructible. You can get it on ebay for about 15€, I got mine in a nearby military shop… actually I got three, just in case.
Careful if you are purchasing this kind of spade, be sure to get the one with the square pin type to block the shovel/pick part in place. The one with the screw tight thingie doesn’t quite do the job as it tends to unscrew and leave you bleedingin no time 🙂
Fiskars, known for its gardening tools, makes my #2. The main thing about this little digger is the material it is made of – nylon and fiberglass composite. Having a tool made of a non-metallic material, means your metal detector won’t detect it, and you can pass the digger filled with scooped earth (and finds) right throught the coil, keeping your hands clean.
I have seen people carrying around a huge number of spades, knives, diggers… etc., but for me these two have proved to do the job.