What's the best working format for video making?
The best format is using a temporal file of lower resolution.

Make a copy of your source material at half size, which will become 1/4 of the weight, then use some format that is compatible with your program and use it for the editing process, when it is ready, relink the videos to the original ones for the final render and done. Make some tests and see if the keyframes are working fine for small clips.

If you only want to change the source material to another format you are degrading your video.