Many people will agree that using a Neutral Density (ND) filter on a drone will often improve the video footage and also still photos (if you want smooth flowing water) but there are many brands around so which to choose?  

But it may be worth recapping why we should use an ND filter. It lowers the exposure in very bright conditions and prevents over-exposure. The oft quoted analogy is that it is like using a pair of sunglasses on your drone! Although the aperture is fixed in most cases, surely, I hear you say, I can just increase my shutter speed to prevent that overexposure. Well yes you can but then your footage won’t look as professional as it could, especially when filming closer to the ground. Counter-intuitively when filming video you want to introduce some motion blurring of the background so that you get a more cinematic look. Most professional movies are shot with the rule that says your shutter speed should be double that of your frame rate So for example if you are shooting at 60fps then your shutter speed wants to be around 1/120 second. And this may not be possible unless you use an ND filter. The effect is particularly noticeable when filming fast and close to a subject.

When I was looking for a set of filters for my new Mini 4 Pro I turned to the brand I trust with most of my drone accessories - Skyreat. They make quality products which are competitively priced. The set I bought was a 6 pack – ND8, ND16, ND32, ND64 and ND128. For my shooting this covers the situations I am likely to need ND filters and there is a bonus of an included Circular Polarizer (CPL) filter which helps to remove reflections and eliminate glare from water, glass and other shiny surfaces.

The Skyreat filter set comes in a nice plastic case (I would have liked there to have been an extra space for the lens hood which comes with the drone) and a cleaning cloth is also included. The filters themselves are multi-coated, scratch resistant and have a neutral colour shift. The frames are plastic but that saves on weight so is a benefit for me instead of aluminium frames which weigh more. They feel nice and secure once fitted and I can highly recommend them.

I'm a diehard Freewell ND filter fan and have them for all my drones and the OP3.  However, I may pick up a set of the Skyreat filters to try them out based on your recommendation.  

I went with Freewell, as I've had great results with them previously.

There are 2 (roughly stated) reasons to get ND filters.
1. For stills: You usually use very dense filters. Like ND1000. So you can get nice blurred motion in your stills (waterfall etc.)
2. For video : You usually use lower densities (ND8-64) and this is done to be able to introduce a slight amount of motion blur, which can look more pleasing to the eye.

Now both can be a good addition to your toolkit, but the second one is often seen as a general 'video improver', which it is not.
Especially with drone footage, where your subject is further away often, the effect is minimal and there are other 'free' techniques you can learn that will improve your footage a lot more than ND filters can.
Think about :
- Storytelling
- Composition
- Using light in your frame
- Using motion to enhance your story
And to make sure you don't get stuttering (which is why people think they need ND filters) you can also incorporate 'free' techniques to make sure this is less likely to happen
- Fly and move slow
- Use multiple axis for your movement (forward and panning and moving sideways) instead of 'one axis moves'.

I have had ND filters for all my cameras, but I found that drone footage usually can do without and the 'free' things I mentioned above are much more important for drone footage than getting a bit of motion blur.

So yeah, for videos taken with my ground cameras, with close by subjects moving quickly through the frame, I see more (bigger) effects and I use ND filters a lot.
But for my drone, I almost never use them.
And TBH I have never seen a ND filter demo with drone footage that proves me wrong. The footage 'without' was always smooth enough for me.
They usually show some ground footage of a moving 'thing' (wheel or something), but that is not the type of subjects drones capture (with me at least) often.

And for brands. PolarPro and Freewell are my weapons of choice.