I compared an Adele Skyfall flac file to the official music video on youtube and youtube's version had a much more crisp, and accentuated recording.
I always thought youtube videos played mp3 files?
The 720p/1080p MP4 container on youtube uses 192 kbps AAC for audio. The WebM (HTML5 version of youtube) container uses 192 kbps vorbis.
This doesn't guarantee high quality though because the uploader could have used a different lossy codec.
Max 192kbps AAC or Vorbis