If the source is below 320 kbps, even if you find the best converter in the world, you'll get only oversampled bitrates. Low bitrates will always produce low quality audio even if you get your files converted and encoded at 320 kbps Difference is not noticeable on crappy hardware as computer speakers etc, but the difference is huge when using high end systems.
Most youtube videos contain audio in the form of 128k AAC. Which actually sounds fairly good. AAC is a much newer and more efficient encoder than MP3. Your best bet, in my opinion, is to download the youtube video and then strip the video, leaving the 128k AAC audio stream which you then save as an "*.M4A". Nearly all music players made in the last 10 years will play these "*.M4A" files. If you re-encode from AAC to MP3 you are going to lose quality, no matter what bit rate you choose for the MP3 conversion. Ffmpeg, free software, will do an excellent job of stripping the video. Code: ffmpeg -i "input.file.mp4" -vn -acodec copy -map_metadata 0 -id3v2_version 3 "input_file.m4a" -i "input.file.mp4" == the name of the input file, downloaded from youtube or whereever -vn == video no, do not pass through any video streams -map_metadata 0 == pass through any metadata (such as tags) from stream 0 -id3v2_version 3 == save any tags as version 3 of id3v2 "input.file.mp4" == provides a name for the output file On a downloaded youtube video, there won't be any metadata (tags), but it certainly doesn't hurt to pass them through if they do exist. And on the original question, I like and use 4k video downloader.
Why on earth would you want to leave your data online at another company??? Simply use VLC, it works absolutely perfct, fast and easy...