320 kbps is the largest and the default here. It only pays off when the source audio is already good — a studio music video, for instance.
192 kbps is the sweet spot for most music: clearly smaller files, and very few people can hear the difference on regular headphones.
128 kbps is plenty for speech, podcasts and lectures, at roughly 1 MB per minute.
A note on quality
YouTube delivers audio that is already compressed, usually between 128 and 160 kbps. Converting that to 320 kbps cannot bring back detail that was never there — it only makes the file bigger. Pick 320 for archiving, 192 for everyday listening.