Should I download YouTube videos in FLV or MP4 format?

MP4 is certainly the most versatile of the two. FLV requires another step to make it readable by video players, whereas MP4 files are already encoded and ready to watch.

Quality doesn’t depend on the formats being used but I find that FLV vids tend to be lower quality, webcam style videos while encoders usually choose MP4 to squeeze every bit of quality out of the source.

H.264 (mp4) can be superior quality to icky FLV