Just my personal opinion...
1) I have not tried them all but I bought Polar Pros. They have the reputation and products I was after at a reasonable price with a good warranty and their staff answered my questions rather than accusing me of being an undercover agent for the competition (LOL). I'm sure there are some other similar filters out there but that was the safest bet for me at the time and I have been very happy thus far aside from the fact that for the Mavic Air they have their ND kits precisely set up so that you need to buy two of them to cover the most common scenarios (not uncommon but annoying nonetheless). They do offer custom kits from their store directly, but as a Canadian that is less convenient. My view is that filters (even Polars) are so cheap relative to the cost of the drone and so important to image quality that there is no point cheaping out. I have the same opinion of memory cards. YMMV.
2) It completely depends on your personal usage and the conditions you shoot in. If you live/fly somewhere that is always overcast, you will probably use the ND8 the most. If you live somewhere sunny or tropical, you will probably use ND16 and ND32 the most. If you fly a lot at dawn/dusk you will want a ND4 and if you fly a lot over snow you might want a ND64. Given that the
MP2 has a variable aperture you have even more flexibility to combine with various ND filters (note that after F4 image quality will begin to degrade slightly at first and then very significantly as you approach F11).
In my opinion an ideal kit is the ND8, ND16, ND32, ND16PL, ND32PL - that can easily be different depending on what you shoot. I never shoot at dawn/dusk for example.