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.
