YouTube presents audio in 2 forms under various bitrates and formats. AAC and OPUS. The Opus track @128kbps (even better than the sometimes available AAC @160kbps) is the highest quality and can be downloaded and played back by demuxing its WebM container to .opus or using a player which understands the WebM container.

This is what I prefer: Downloading original OPUS audio, demuxing to .opus, without reconversions using a bash oneliner. Highest Quality you can get from YouTube.

Appreciate the minimalistic UI, no frills. 170 MB for a YouTube MP3 Downloader seems a bit steep though.