On YouTube itself the "best" is 720p video nowadays, and it doesn't seem to include the actual best audio. The -f bestaudio+bestvideo flag grabs the actual best available from YouTube, but because it comes in separate streams it requires ffmpeg or avtools to be installed to mux them together. To what extent YouTube's 1080p is better than its 720p is debatable, I suppose. The quality looks fairly similar to me and the bitrate of either option is a tad on the low side. I think 1080p is maybe a tad sharper on a 1980x1200 monitor. More important is the audio quality:
139 m4a audio only DASH audio 49k , audio@ 48k (22050Hz), 2.27MiB
171 webm audio only DASH audio 124k , audio@128k (44100Hz), 5.02MiB
140 m4a audio only DASH audio 129k , audio@128k (44100Hz), 6.04MiB
172 webm audio only DASH audio 187k , audio@256k (44100Hz), 7.63MiB
141 m4a audio only DASH audio 256k , audio@256k (44100Hz), 11.99MiB
This is the audio in the 720p file:
$ ffmpeg -i QWOP\ \(Elders\ React\ -\ Gaming\)-NCTL7BebBbE.mp4
[…]
Metadata:
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 191 kb/s (default)
And this is the audio for what may be the best (in past listening tests I've preferred Vorbis, so it's conceivable that I'd prefer the "WebM" option):
Duration: 00:06:34.18, start: 0.000000, bitrate: 255 kb/s
Stream #0:0(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 253 kb/s (default)
It's a pity YouTube makes you jump through such hoops, and that its video quality is so much lower than on Vimeo. On the plus side, youtube-dl also supports Vimeo and various other sites I use.
PS Ignore any of the lower quality audio files like the plague. I've heard they cut off at 16 kHz instead of 20 kHz.