Obviously it depends a little on how and what you shoot as to which ones and how dense you want them, but my views on the three main types for stills use from my own research are:
ND: Darkens the entire frame, so useful if you are in bright light and want a longer exposure, e.g. to get some motion blur in water, or to get a correct exposure at a wider aperture - the
M2P seems sharpest upto around f/5.6 or so, meaning a 2-stop ND might make the difference between a softer shot (with more DoF) at f/11 or a sharper shot at f/5.6. Or you can tweak the ISO a bit to help achieve the same DoF control, and if you shoot DNGs then you'll have more latitude in post too.
GND: A little more tricky as they are generally going to have the point of transition dead centre - e.g. you can only really shoot with the camera perfectly horizontal with the horizon in the centre of the frame, then crop as required. Can still be very useful if you have to deal with a large amount of dynamic range between the sky and ground though. I'd actually like to see some GND sets at 2/4/8 stops with the transistion point at (say) 33%, 50% and 66% of the way through the frame for a bit more flexibility for rule of thirds composition options, but I'm not sure if there would be enough demand to make that a viable product line.
CP: Even more tricky, as they depend on the angle of the sun so will change the level of effect as you rotate the drone. Worth it though, as they'll provide more saturated colours and cut down on reflections from water, etc. Ideally you need to pre-visualise your shooting angles and set the polarizer accordingly, making multiple flights with different CP settings if need to be to cover off different angles to the sun.
Andy