Which bitrate sounds best?
Bitrate is how much data each second of audio gets. YouTube's own audio is roughly equal to a 128–160 kbps MP3, so 320 kbps won't add detail that was never there, but it does avoid a second round of heavy compression. For music, 256 or 320 kbps is the safe choice.
For podcasts, audiobooks and talks, 128 kbps is indistinguishable from higher settings and saves more than half the space.