I have subscription to all three. What is good and bad
1. Spotify.
Good - The best designed interface, miles ahead of apple and google. The playback mechanism is well designed. Spotify pulls the music file from the server and plays it directly to the playback device. There is direct communication between the playback device and the spotify server. Spotify shows the same song, same information on each and every device.
Bad - Honestly just the lack of lossless and hi-res. The day spotify comes with this, there won't be even one advantage of having apple music.
OS support: Windows (official), Linux (official via snap), Mac (official), BSD (3rd party)
2 Apple Music
Good: Hi Res available for few tracks. By Hi-Res I mean anything sampled at 96 kHz and above. Availibility of lossless content is also plus.
Bad: Bad design. Mediocre design of playing at fixed sample rate on macbooks, mac mini, apple tv. Devices played at fixed sample rate only when the dinosaurs were alive but apple does it even in this age. Correct design only on ipads and iphones. Music is played on the end device using multiple hops. First the device playing apple music pulls the song from the apple server and then pushes it to the playback device. If the playback device is not the one running apple music app, then you are losing quality. Quality sucks on non-ios devices like macbook, mac mini, apple tv, windows/linux laptops. Extremely difficult to play bit perfect music. Also every device running Apple music app will show different song, different information. Spotify does this beautifully allowing you to switch between devices without losing the song being played and the context. There is just two use cases where apple music beats spotify in quality. That is when you are running apple music app on an ipad or an iphone with a good dac connected to the usb-c port. The moment you switch to a macbook, mac mini, laptop that advantage over spotify is lost. There is one more use case for apple music. That is when you are using Airport Express and the content from Apple Music is sampled at 44.1 kHz (which happens to be the major content that AM has). AFAIK only
@sachinchavan 15865 and myself use airport express.
OS Support: IOS (official), OSX (official), limited to 44.1 kHz on Linux running non-official shairport-sync
3. Youtube
Good: User content
Bad: everything else
OS Support: Official Via Google Chrome brower or google cast plugins. Quality will never be at par with AM or Spotify as there is no native OS support. Plenty of non-official hacks which download the content and then do the playback.