Probably PolarPro (< link to their M2 filters) as they support the drone market very well.
They have great optics, coatings, and importantly support . . . not that others don't have support, or really need it, but I've seen posts here of problems very rarely, and PolarPro really sort it out quickly for their clients it seems.
For video, I personally would only get the plain ND range, say 4 to 32, or 64 if you film in places with snow / white beach sand on sunny days.
PL filters if you really need water / reflection reduction, and are prepared for the extra hassle using them on drones.
An NC-UV for photography if you feel some times need little help with better colours, less haze, protection for the camera.
Not sure if PolarPro do full sets like some manufacturers or if you need to buy some separately.
I went with NISI, as they are probably one of the biggest camera filter suppliers Worldwide.
Their optics and coatings are also 2nd to none, they had / have great sets available.
Mine for the M1P aren't made / supplied any longer, but they do have the M2P . . .
View the full range of NiSi Drone Filters products at NiSi Filters Australia. Free shipping on orders above $69.
I have a natural night filter of theirs for mine, but we aren't allowed, but biding my time for some photos if / when a suitable safe time comes to take a flight to 50m straight up and down to test it vs none fitted.
You don't need a filter for bright sunny days. You don't need a filter to take videos.
Are you looking to use a filter to change the video in some way? You can get those that filter out light to slow the shutter speed down, but it doesn't change the "bright sunny" part--it just slows down the shutter. Some folk do this to get a more "motion blurred" cinematic look. Is that what you are looking to do?
There are filters that polarize the light, which tend to cut down reflections (much like some polarized sunglasses do). And they might saturate the colors somewhat. Is that what you're looking for?
I'd say they read this somewhere and want to use filters, or think it's necessary.
Whether the science is understood, not sure.
They're here to learn, so . . .
The only reason to use filters is generally for video, to slow shutter speed to roughly double frame rate.
This smoothes out the look of video, which the human eye finds more appealing / viewable.
Eg. 1/50 shutter for 25 FPS, 1/60 shutter for 30 FPS etc.
Not sure if the OP wants the 'smooth to the eye' feel of motion blur, but that's what most drone pilots want in their video footage.
Sure, you can use ND filters for photos (same sort of motion blur on waterfall / waves etc), but the drone movement works against that to provide clear sharp photos, so best left off, or use a simple UV filter.