Online converters always cause bad output quality. I suggest this 60fps downloader: http://www.videoconverterfactory.com/tips/60fps-video-download.html It converts YouTube to MP4 and MP3 and the output file will not be compressed badly. At least I don't see any difference.
If you want to save YouTube to MP4, KeepVid should be your first choice. keepvid.ch/en/youtube-to-mp4