Don't mean to kick a dead horse here but haven't found what I'm looking for and in the essence of time...
What is the BEST over all settings on GoPro for smoothest video?
4K, 30fps, Wide, Protune, ISO 400, Flat Color, Native White point.
This requires a top of the line SD card.
It also means you will need to color grade your footage in Final Cut or Premiere. If you don't want to do that, use GoPro Color instead of Flat. You still may want to adjust contrast and saturation when you edit your video. If you don't want to do that, then turn off Protune.
4K is great even if you are delivering in 1080P. Even if you take your 4K footage and export it out at 1080P, it will look better than footage captured at 1080P.
IF you think you might want to do slow motion, then you will want to use a lower resolution (like 2.7k) at a higher frame rate. That will allow you to slow down the footage in software, without dropping below 30 (or 24 fps). TV is 30 fps. Movies are 24. If you want your edited footage to look "cinematic" then you will want to letter box and conform your video to 24 fps before you export.
The nice thing about 4K and 2.7k is that you can also reframe shots without resizing pixels. As long as you make sure you don't crop the footage below 1080P, then you can get really good results.
A lot of people will say that you should shoot at a higher frame rate for smoothness, but personally I find 4K to be so much better that I just like it. I'm willing to be stuck at 30fps for the sake of 4K. Can't wait for the GoPro 5 that will let us shoot at 4K 60 fps.
What is the best ISO settings?
Almost always set your ISO limit to 400. If you let it go above that, you will get noise in your footage. If you are shooting in low light, you may have to set the ISO higher. In which case use Neat video to remove noise. It is by far the best software out there for removing noise.
With or without an ND filter or Polarizer filter?
Your goto filter should be an ND4. Circular polarizers require that you rotate your filter to the right angle to match the light (they work best when the light source is 90 degrees from the lens). That means that when you are flying a drone around, twisting and turning, the CP may not give you consistent results. If you have scouted the location, planned out your shots, taking the light into consideration, then you might be able to know ahead of time exactly how much you want to rotate your CP.
Or you could just use an ND4 with a CP. I don't think the CP will noticeably hurt your footage, but it will improve it in some cases, at certain angles.
For example, is the 4k @30 frames with better resolution or 720 @120 frames better for better for overall video?
I know each situation is different in as far as lighting conditions etc. I'm looking for the base line and saving some hours of experimenting and uploading.